There are many things that could be going on, but you won't know for certain without a visit to your vet. Glucosamine, chondroitin and MSM are good supplements for maintaining joint health, but they are not curative substances. Your dog could be suffering anything from simple arthritis; to hip dysplasia; to a pulled muscle, tendon or ligament; to a ruptured spinal disc; a tumor or cyst in a bad place; or any number of other issues. She's obviously in pain, so get her to the vet as soon as possible. Hopefully it's simple osteoarthritis or a soft tissue injury that will require only anti-inflammatory meds to get her back on her feet again.
I basically agree with Jaybay.
However if you are giving Glucosamine supplements to her, for a 90lb dog 500mg is probably not enough. It is the dose I give mine, and she is a 28lb dog. Yours may need 1000 or even more mg Glucosamine daily. The safest thing to do about that is have a word with your vet about it, if you are unsure.
I honestly did find it worked well for my dog, very quickly and very efficiently. But all dogs are not the same. Yours could have very advanced problems, or like Jaybay says, other issues that need a tougher approach.
I do hope she has some improvement very soon.