Grandma feels miserable but can't elaborate. How do I handle this?

Grandma has been moaning and groaning more than normal. She's not eating/drinking like she was. She's hungry but can't eat much and can no longer finish her favorite meal. This started sometime on the 25th after having eaten quite a bit of goodies on the 24th. When I ask what is wrong she says, "I don't know." Then she goes on to say that she is miserable but doesn't know why. She can't give me anymore information than that. This is the third year in a row that she has acted oddly around Christmas and it is also the the third Christmas in a row without Grandpa. She last saw the doctor in November and was in good health. Her only medications are a blood thinner due to DVTs and medication for an underactive thyroid. She just finished up Bactrim DS on the 27th for a very mild UTI.

Do I take her to the doctor or wait this out for a few days to see if it's more of a seasonal thing or possibly related to the Bactrim? She says she doesn't want to go to the doctor. However, if she continues on like this she will have to go if she's going to continue to live with me. I need to know what's going on here. She likely has (mild) dementia, but we've never gotten a diagnosis for it.

On the 27th, she didn't even seem to know her two grandchildren that were here. She's had family over and she did very little talking. She seemed to wear out quickly. She seemed interested in her great grandson (he's 15 months), but if she wasn't looking at him she was staring at the TV, seemingly ignoring all the conversations around her. Years ago she used to be a chatter box, never could get her to shut up. Now, the only time she really talks is in the car.