I don't buy that it's nothing as your vet says. If she is limping, she is in pain.
How does she walk on the grass vs the sidewalk? In the 3 months has it gotten any better?
How long ago did they do the x-rays and how extensive were they? If nothing is showing on the x-ray, tht is a very good thing.
Here's our story- please keep in mind I am NOT telling you this to scare you. One year prior to Polli being diagnosed with osteo, she started limping pretty severely. (Sept 07) It was a horrible limp. On grass it was better. We had a set of x-rays done nothing showed up. We waited another week and under a general anethesia we had a full set of x-rays done, again, nothing showed up. After about 2 months, it got better. In the course of a year it would flare up, we would rest it and it got better and go away until Aug. 08 where it appeared and did not go away. it wasn't too bad and we weren't too upset because we had it x-rayed and it was clear.
Then Oct. 9 08 she tripped and fractured the leg. The rest is history.
I know you are scared about the cancer. But before you panic, rule out other things. She may have a tear, or a corn that needs to be removed. Corns are very, very painful.
Both Teddy and Chloe have had limps. Chloe from an old injury and with deramaxx and rest it resolves itself. Teddy needed acupunture for months and that really helped.
Does your vet see a lot of greyhounds? If not, you may want to see one who does.