What can I do to help my mom who was very close and is now treating me, her only child, as a stranger since my father died last month?

You are right in thinking that something is amiss. If your dad was 91, then I assume your mom is at least late 80s if not early 90s herself. That generation is way too trusting, naive, and gullible. (Going through the same thing myself with my dad.) They believe that everyone has their best interests in mind and are too easily swayed. You need to do some digging and find out who she has been in contact with, and I think you need to act quickly.

It's possible she's being scammed or exploited, maybe by someone who actually is an attorney, or by someone who claims to be giving legal advice. If her husband died a month ago, she probably has had necessary contact with some type of attorney recently. She and her husband probably had a will drawn up with an attorney, maybe a trust as well. This attorney might now be helping her create a new will. Do not assume the attorney is your pal. Do not assume the attorney is doing what's best for your mother, for you, or for your family. The one thing you can assume: an attorney will strongly suggest and actively promote the most expensive legal solutions to your mother and anyone else in the family. This would include the attorney making himself executor of a new will, the attorney naming himself in a third-party trust, etc. Go visit mom and do whatever you have to do to get a look at paperwork--will, trusts, house title, etc. If you must sneak behind her back to get access to this info, so be it, you're doing it for a good cause. At least get contact info for any lawyer involved. If a lawyer actually is telling mom to get rid of you, that's a very bad sign. But it could be that's just mom's bizarre interpretation.

Oh holy mackerel - get round there, fast. It really does sound as though some low-life has moved in on her. In her state of profound grief and dislocation she is terribly vulnerable; and you need this cleared up.

You may need to be firm at first. Don't plead, insist on an explanation.

If Mom has caller ID on her landline, you should be able to see who called her. Of course with a cell you see who she called and who called her.

I hope you have POA. Makes life easier. Ciuld it be that caring for Dad you didn't notice the early signs of a Dementia. Chalked it up to old age? Mom could have a Uti, could be taking her meds wrong. The death of Dad could cause a depression. She could not be drinking or eating properly. Dehydration can do a number of things.

If I were you, I would definitely visit and see what is going on. If she still is not herself, take her to her PCP for a good checkup.

My first thought is your mom is depressed.
Your dad died, but her husband died...A man that she has probably been with for more than 1/3 of her life. One that she loved, raised a family with, cared for and now....nothing.....

I would go by for a visit.
Go in and see how she is doing.
If she looks at all "off" soiled clothes, hair not combed, anything out of character for her I would be very concerned.
If necessary call 911.
Look into a Bereavement Support group you could go with her. Although there are support groups for people that have lost spouses.

My thought is she may have decided to just "give up"....

Make sure mom isn't sick, first thing. UTI's are so common and can completely change a person's demeanor.

If that is not the problem, just give her some time. Does she have a neighbor you can ask to kind of check on her, daily?

She's probably grieving, and grief does funny things to people. It's a huge change to go from living with someone for many years and then losing them and having to find your 'new normal'. Maybe she just wants space.

When my daddy passed, my mother shed zero tears. She also didn't really acknowledge to us kids the grief we were feeling. It was weird, but not out of the way 'abnormal'. It just was what it was. She didn't want people around her either, and slowly she moved on to a life w/o 24/7 CG.

The fact mom states she spoke to an attorney, yet doesn't have one is troubling. Has she shown signs of dementia herself? Maybe time to focus on mom for a bit.

I’d reinforce Beekee's post. Where I am it is still common to post death notices in the local papers. The bereaved are often approached by everyone from real estate agents assuming that they are going to downsize, to charities assuming that they will rewrite their will and might leave a bequest, to financial planners offering free advice. Scammers are less common, but they can and do happen. Don’t accept that “you don’t need to come today or ever again”, keep going until things settle down, and keep your eyes wide open. If you need to start a conversation, say that you have been approached yourself, or that you have been told about this happening.

G see your Mom. She's probably on an emotional elevator. She may not even remember saying that to you. But if she does, ask her why she said that. Don't hold it against her. Just show up. Especially if you know she is limited in what she can do.

Your mom is most likely experiencing dementia. I would try showing up as if you never had the conversation and see how she reacts. If you notice other signs of dementia, take her to her doctor and explain. She needs evaluation. It's often most difficult for people close to the person experiencing early dementia to recognize it. If you have a friend who has been through this with their own parent, you might want to bring them along and see what they think.

Offer to take her for the usual shopping trip. Check for signs of self-neglect - unkempt appearance, unkempt house... she may have depression and wish to join your dad.