New here, have lump problem with common garter

[guidofatherof5]

Hi, I am new here I have 4 garters 2 of which are common garters and 2 are western terrestrials. Lately one of the common garters has been getting a lump right above its cloaca that will not go away and has been getting bigger. The lump is somewhat soft. The snake has been eating regularly. It has been eating mice and fish. It only eats mice when they are rotting.

Welcome to the forum.
I have to ask, what do you mean by rotting?

Has the swollen area had any drainage? Is the snake still pooping? Does this area around the cloaca have a head/scab to it.
If you have access to a good reptile Vet this may need looked at.
It could be a minor subcutaneous infection or something more.
If possible a photo would be great.
Once again, Welcome and good luck with your sick snake.