She is 85, has lymphoma (but is healthy), and widowed 3 yrs ago. She is forgetting things, losing things, distrustful of everyone, kind of angry, and overwhelmed about the slightest thing. She doesn't understand a lot of financial things any more either.
Take her to her Dr. and have her evaluated for dementia. But at 85 it could just be age-related memory loss. Regardless, I'm sure she doesn't feel well when she becomes overwhelmed easily and maybe there's a medication the Dr. can prescribe that will help keep her anxiety under control.
Do you help her with her financial stuff? Now would be the time to go over everything with her so you can step in when it's time. Do you have POA? If not, get it now.
Most folks here know how difficult it is to watch an elderly parent decline. It's definitely sad and scary. I hope you come back here for support. You're not alone.
"She has lymphoma but is healthy" tells me you may be in denial about her condition. You need to know what stage she is in, what drugs she is on and what side effects to expect. That way you know the difference between an approaching dementia and a temporary effect of chemo. Have a long chat with her MD.