My dog is sick and I don't know what's wrong!!?

My dog is a six month old blue pitbull and she has strange symptoms and the vet doesnt know what's wrong. It's very random somedays she's just fine but somedays she's in a lot of pain and can't walk. She is urinating and defecating like normal and she eats normal also. On the days she's in pain she can barely walk and she shakes when she trys to stand. You can't touch her because if you do she screams. No she doesn't yelp she screams. I don't know how to help her. I'm scared.