Small bump..

I have noticed that there is a small bump on my 5 month old cat's back left leg (about his thigh area). It is under his skin and about the size of a bead or maybe half the size of a marble. He is an indoors only cat (so I don't believe it is a tick..) and is currently updated on all his vaccanations. His last two vaccatanions were in his back leg (although I can't recall which one) and I wonder if it could possibly be some sort of reaction, although his last shot there was about a month ago. Is this something I should worry about and take him to the vet for?

One of my mom's elderly cats has this too... vet said it was a cyst from an injection. It would be wise to have it biopsied... but I don't have room to talk, we haven't yet.