My cat has developed a couple bumps of varying sizes within the last week or so. They are on his sides and some little ones on his inner thighs and one on top of his head. He is 16 years old. I thought maybe he was having an allergic reaction to a flea bite or something similar as I think I may have forgotten to give him his frontline last month. I of course have since administered it, and recently gave him a bath to see if that would help, but there is one bump that has just gotten larger and larger. He doesn't seem to be bothered by it at all though. I have been putting an antimicrobial cream on it to try and make sure it doesn't get infected. Should I be worried or should I wait to see if it goes away? He is an indoor cat, but the area he stays mostly is an enclosed porch which has a fish pond built into the ground that I have seen him drink water out of occasionally. I just put some things around it to try to block him off from it in case that he was bothered by something in the pond water. I don't have a lot of money so I'm hoping I don't have to go to the vet but I don't know if I should be alarmed. Any ideas as to what it could be?

Any bump, but especially one that is growing should be looked at by the vet. You mention putting antibiotic on it to keep it from becoming infected... this suggests that it may be hairless, even 'raw'? Even more of a reason to seek a better opinion on it. There are a variety of malignancies that affect the skin that need to be detected and treated sooner than later.