My mum asked about negative thoughts. Any advice?

This world is changing fast. And it may be hard for her to keep up.. So keep reassuring her it's ok.. This too shall pass. She may be wishing for the better good ole times., that is ok too,..
So, maybe this is the time, to say to each other, a good memory or thought. Tonights dinner was good/ The sunset was nice. Something positive. a good note.. perhaps good deserts that you two shared making at one point.. Perhaps it's a good time to make it again..
Find the silver lining.. and say one positive thing. anything. about anything. maybe gardening, or a favorite hobby... I am almost tired of watching tv.. but I found a good cooking channel.. So now I have a captive audience. I can make whatever I want, and I will let them know when dinner is ready.. Hubby asked what's for dinner? I said, whatever I make.. :) He replied... ok...
I hope your mom is close to you. Daily connection is important for her. REassure her, she is ok.. and it's ok to feel that way... If it lasts too long.. she may need to have her doc prescribe something, prozac or a similar antidepressant, small dosage..

How very sensible and insightful of her to ask the question!

She understands in her head that all is well, but in her heart she doesn't feel right - do you think that's about it?

Is she in good health, generally? - do you agree with her that she is?

How is she coping with the current worldwide health emergency? I'm meeting a lot of people who are feeling very shaken by it.

Do you think she is depressed or has anxiety? Has she spoken to a therapist about her feelings? Right now is a horrible time to schedule face to face appointments but there may be online appointments available. Google mental health appointments in your area and see what pops up.

Or start off with her primary doctor and see what they recommend.

Best wishes to you.

Your mum sounds depressed. Has she seen her doctor lately?