I love garters, and think a lot of people pass them by without realizing how great they are for a first snake. Is your garter WC or CB? How old is it?

It's hard to say with the info given, and it could be quite a few things. The first thing that came to mind was broken ribs that have healed, as it is a relatively common occurance in snakes of all sizes. I wouldn't be too concerned with it unless they start to change (get bigger, more show up, abscesses etc.).

On a side note, can I ask why you choose to feed your garter fish? Garters (especially wandering garters) have quite a varied diet in the wild, and therefore are very easy to convert over to mice. Fish (especially goldfish) are not the best feeders for snakes. They can harbor parasites, and if you are feeding fish worth feeding to your snake, it is much more cost effective to feed f/t mice anyway.

-Evan