It amazes me how differently people react to similar situations. I found a lump on Wickett right after having him at the vet. The lump was no where near his injection site but he was scheduled to go back in 3 weeks to have his teeth cleaned. I was lectured, guilted and chastised for not contacting my vet immediately. When I did call my vet I was again lectured because he told me that unless there were changes it could wait until his next visit in 3 weeks. For me, it was in a spot that I don't pet with any concentration so I have NO idea when it developed. It's about the size of a pea and even my hubby can't always feel it... but I can now if I'm looking for it.
All that said, he was in for his cleaning last Friday and the vet said it was probably scar tissue, to continue to keep an eye on it and call them if I find any changes in size, location, mobility, or pain. I'd keep a very close eye on it and her. If anything chanes between now and the 13th then I'd call... if not, I'd just wait it out.