My daughter's cat has had a bump on her left hip and she finally took her in for a check on it. After a surgical biopsy it turned out to be devastating news. A fast spreading cancer. They suggested removing her entire leg but with no guarantee that the cancer hasn't already spread. Just thought I'd pass this along as it is actually quite common to felines and certain injections given in the left hip.
About 1 in 10,000 cats have a tendency to develop fibrosarcomas (cancer) at injection sights. Vaccines are the most common trigger, but we see them after antibiotic injections too. The FELV vaccine is the biggest offender and it is usually given in the left hip/leg. Fibrosarcomas are firm, rapidly growing masses. If it is a fibrosarcoma, simple removal of the mass is surgically difficult and ineffective because it comes right back. Amputation of the entire leg is usually needed to get all of the tumor. Amputation provides a cure in about 75% of cases. In the other 25% we find that the tumor is already spread either up the leg or to other parts of the body. If money is not a concern then it is best to do an MRI before surgery to determine how deep the tumor is invading.