Lump at injection site

[norwegn113]

The lump is completely normal and will go away in time.
Most antibiotics require IM (intramuscular - in the muscle) injections. You did SQ (SubQ/subcutaneous - under the skin). It won't hurt your snake. You just injected under the skin rather than into the muscle. It will be just a little more slower to absorb than if you injected directly into the muscle. The bubble is just the liquid sitting under the skin. (Think of when you got a TB skin test - it was injected under the skin and produced a bubble)

Next time, just push the needle down a little more deeper if you want to get into the muscle.

You are absolutely correct. I had the same thing a couple of times my snake moved and the injection went sub Q . Nothing to worry about, the lump will go away.