Honey, it is so good of you to care about your grandmother. Getting old and frail is hard..learning that you must depend on others is harder. I am not sure what you mean by mental problems, I am going to guess that you mean she has some form of dementia. Being in a new environment can be confusing and depressing for any elder, but for one with dementia it is even more so.
I hope she has seen a doctor and her health problems are being managed. Keep in mind that some medications can cause mood problems and so can certain vitamin deficiencies such as B-12 and Vit D. Thyroid levels need checking too. If she seems depressed as it sounds like she is by your description or if her behavior is unusual this should be brought to her doctor's attention. She may have an infection or a deficiency as mentioned that could be detected by blood tests and a urinalysis and of so, she could be happier.
In the mean time, try to keep her as active as possible, try to get her into the fresh air as much as you can but she should see her doctor about any sign of depression. It is good that your uncle has taken her to live with family if she couldn't live alone (and at 90, not many can) but it is still hard on her.