I sure hope that Maggie is doing better and you've gotten some helpful info. I don't want to alarm you, but I will tell you what happened to me ( and Brody )
I took my seven year old to the vet, wondering why he was limping and not able to put pressure on his rear leg. He was little overweight and I thought maybe played a little too hard. The vet felt around a little bit and determined that he had a torn ACL. She said she was worried about doing surgery on it, she wanted to wait for him to lose 10lbs before she would do it. I fed him a strict diet, exercised him moderately, blocked him from the steps (sleeping downstairs with him of course ),and kept him from playing with his younger sister.
After a good amount of time passed I could tell that things were getting worse. Not impressed with my vet, I took him to a vet that a co-worker recommended. He felt around, unable to determine what it was, and immediately scheduled exploratory surgery the next day. After the surgery he called to tell me that Brody has a tumor in his knee. In the couple of months since my visit to the first vet, the mass had grown too large to operate on. It was in fact malignant. We had a great six months together after that and then he was gone.
If my first vet had diagnosed the condition inititally, it probably could have been taken out and treated.
Just make sure you've got a good vet who covers all the bases. I feel guilty I didn't do that sooner. I hope tHat you get good news and that Maggie is feeling better in no time. Take care.