Mistakes happen. And sometimes septicemia can just creep up on even a well maintained snake.

My 18 year old just had a case of septicemia a few months back. Clean bedding, good temps and humidity. Unfortunately, this is my largest snake who also refuses to eat anything but live. So she probably picked up a bacterial infection from a minor rat bite I missed (which is thankfully rare). I am always careful to watch and clean out any bite wounds immediately to avoid this situation when I can. But it may have just been coincidence or a bacteria the rat was carrying and as a live rat, wasn't killed off in a freezing process.

As a heads up, antibiotics may keep your snake off food a while longer. Orals are worse on that than the injectable.
I would still offer food just in case of you have another snake you can drop the food to if this one still refuses.

I've had good recovery time on a septicemia inflicted turtle on that same medication. Unless it's a very resistant bacteria, that should do the trick.

Hope your noodle gets well soon!