From my understanding, regurgitation happens when the prey is too large, the temps aren't warm enough to allow digestion, the snake is incredibly stressed out, or there is some underlying pathology.

A small mouse probably isn't too big so I doubt that's the issue. What are your temps and humidity and what are you reading them with? And does the snake have proper security spots?

I'd have to agree with Boanerges though...a trip to the vet is your best bet.

Dennis