Burmese pythons can be a bit...uh...nippy; more so than other large snakes. I was bitten in the face by a medium sized one several years ago at work. I suppose I looked tastier than the rats I was offering!
How much did you warm the rats? Pythons have an additional "sight" using those pits to detect heat. If your face was warmer than the rat, your face would really stand out in the python's "vision." In that case, the python might think that you were the rat that he was smelling.
Bill