There are a few possibilities. How old is the dog?
It could be a ruptured ACL...very common in our breed...and only surgery will help. It could signs of Hip Dysplasia, and you need to have it diagnosed with an x-ray. It could be a sprain or a strain? Rest is needed and no activity. It could also be the dreaded but very common for this breed osteo-carcinoma (bone cancer).
This dog needs to be seen by a vet. See if you can beg,borrow or sell some of your things to round up some cash.
Gina