One of my snakes does that, it's like she's trying to make sure the mouse is good and dead before she swallows...as in she is suffocating the (already dead) mouse. It could be that, or it could be because you are in the room looking over it and stressing it out. <--not sure if you are in the room so that's just another senario
If you are continually checking on it, just leave it be for a couple of hours (no peeking!) and it should swallow the pinkie down. If it's still like this after a couple of hours then there *might* be a problem so call the vet. Hope it all turns out ok.