The only things I can think of that would cause the problem are some slippage between the bit and collet (any signs of that on the bit shank?), a dull bit, too slow a feed like David said, or a bad bearing on the router (but you should be able to hear that). If it's the bottom bearing then running it for several minutes without doing any routing should still cause it to get hot.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.