Ditto on everyone else, you want to be feeding smalls at most not mediums yet.
What temperature is the rat when you offer it?
An animal usually wont go after something it doesn't think it can take, your snake doesn't know the rat is already dead and wont attack it unless it deems it the correct size for food. Well it may "attack" but once it wraps and figures out how large the rat is would leave it alone rather than swallow.
But it might "hesitate" if it's having a hard time recognizing it as food because is smells funny or is not hot enough (which is hotter than it would naturally be).