Pelvis fracture.

Dear Dr Marie,
My dog currently has a broken pelvis that is not mending.
It started as a slight limp for a few days which didn't seem to be causing her any pain so we rested her thinking she had been running round too much on her walk the previous day as she is very energetic. Then when we woke her in the morning she was in pain and couldn't put weight on it so we took her to the vet. He x-rayed it and found a break in her pelvis.
Due to the location he said that there was nothing he could do except give her a shot of steroids and calcium supplements to encourage quick healing and that she just needed to rest and for it to heal naturally. That was about 2 months ago and its still not fully healed- it keeps improving to the extend that it doesn't hurt and she is starting to put a little more weight on it and then getting worse again.
We are going to take her back to the vet tomorrow but I also wanted a second opinion as last time we were there he said there was nothing he could do.
We don't know how she broke her pelvis in the first place- she hasn't been in an accident that we know of (i.e. outside of the house) and as I mentioned it seemed to start as a slight limp then 2 or 3 days later was much worse. When she is well she is a typical terrier in that she is very active and very bouncy- I wondered if she could have injured her pelvis out on a walk or playing with her dog friend in the house then kept bouncing around and exacerbated it until it broke- is that a possibility?
She is a rescue dog and we know a couple of things from her history that perhaps could have contributed to it:
1. She used to live on the streets and was hit by a car which fractured the same back leg and it was not set by a vet so it healed crooked so that leg was already damaged and a little foreshortened so she often alternated between walking on 3 or 4 legs but it did not seem to ever give her pain.
2. She got taken into a dog pound and was very close to starving to death as she was the smallest dog there so wasn't getting enough food as they were fed communally (her ribs and hip bones etc were extremely prominent). I don't know how long she had been there and in that condition.
My vet mentioned that this could have led to her having weakened bones.
Can you suggest anything new for us to try to encourage the bone to heal? Since she did the injury we have been resting her not taking her for walks just outside to go to the bathroom. A local boarding kennel has a dog swimming pool- would it be beneficial to take her swimming or is there anything else we can try?
Thank you for your time
Best regards
Ruth

Dear Dr Marie
Thank you for your reply. Minnie is having a good day today-she is still limping badly but she is much perkier than yesterday- I just wish she wouldn't wag so much because I'm sure it can't be good for her hip!
I forgot to say that at our original appointment the vet did also mention that as well as an accident it could have been called by a bone tumour but it was hard for him to tell on the x-ray because she's so small (she's about 11 punds). He did say that if it was a tumour we would know because she would deteriorate very quickly and warned us that he would not be prepared to operate on her to remove the tumour because he did not think it would succeed due to her size.
After the first few weeks when she started to improve we assumed that this ruled out the possibility of a bone tumour. Do you think it could still be a possibility? She is better now than when it first broke not worse but it is not healed.
Would it be a common concern amongst vets carrying out such an operation on a very small dog or would it just be a case of us finding a specialist orthopedic surgeon to do it?
Thaks for your help
Best regards
Ruth