New here, have lump problem with common garter

[guidofatherof5]

I am doing that right now and I noticed that the lump has moved about 3 inches toward the snake's head

You've got my interest. Moving back towards the head doesn't sound like something to do with digestion. I think any mass/growth/tumor would stay in a location and maybe grow but not move. I'm leaning towards a parasitical problem. Some forum members have reported parasitical worms under the skin but never reported them changing locations. I hope you can get a photo posted soon.
Is this lump on the side, top or bottom? Is it a hard mass under the skin or does it feel more to be fluid filled?