Most likely from prey rather than snake. Blood can seep from head, bite wound, even anus. Snake scales are pretty tough so a tiny bit of kicking will usually not result in an injury. Not to say that snakes are impervious to prey injuries, because they defiantly are not.

I had a snake that was bitten by a mouse and ended up with an infection on the face at the wound site. He ended up having to have a bit of surgical procedure, antibiotics, etc.

To avoid pre-strike bites from prey, I now place the prey in a close quarters container and allow my snakes to strike from above. This way, the snake is not exposed to the rodent before he is actually ready to make the kill.

I always have a good magnifying glass handy to really give my snakes a thorough look over (especially after a shed), so you may try this if you are concerned. When rodents are kicking if you apply some pressure, the prey will usually stop.