I Don't Know What To Do With Her?

My girlfriend has been crying a lot lately. She's becoming more reclusive. We rarely go out in public anymore on dates. Whenever we are outside of the house (we don't live together, so I usually go over to her place) she stays very close to me and holds around my arm tightly. She's been sick a lot lately, and she's becoming pale and physically weak. I'm the only person she talks to now, but i'm the only person that knows she's depressed. All our friends think she's either busy with work/school or sick,. How do I get her back on her feet? I don't even know why she's like this. She's been acting this way for almost a month now. I asked her many times what happened to make her like this, and she never gives a coherent answer. She was such a happy girl!