Can a mass growing inside the uterus of my 94 year old MIL have the same mental effects on her as UTI issues?

I do not at all understand "she has a POA that is a failure at helping". Do you mean that her POA cannot act for her because of being our of state? As long as she can act for herself no POA is needed; the POA will be called if there is an emergency and there needs to be emergency permissions or decisions.

You say she is making changes to POA and accounts and wills. In the case of POA and wills she would be examined by an attorney for competency with each change she makes. Hopefully she is seeing that all accounts have a POD (pay on death) beneficiary, OR is listed in her papers and has beneficiaries listed in her will.

No, a uterine tumor is not known for causing changes in mentation. The two are likely unrelated.

I am uncertain your relationship to this person in familial connection or in terms of locale, or your need to know in this instance, nor even your questions, other than any possible connection between mentation and tumor (which is unlikely). As you are not her POA there may be no issues you need to address.

There may of course be worries about any diagnosis, any surgeries planned, aftercare, ongoing issues in future, and there is likely to be a good deal of anxiety for this woman, which often can look quite a lot like dementia in some people and at some times.

Am wishing her the best.

Here is a post from March of last year that will give us some back story:

https://www.agingcare.com/questions/elder-placed-a-pfa-against-the-family-member-who-has-cared-for-them-473674.htm

I’d suspect some dementia. Plus anesthesia in the elderly can cause cognitive issues. She probably shouldn’t be living alone now.

You should have a Come To Jesus meeting with the POA, who if it’s one of her children, could be having cognitive issues also.