Could be a number of reasons...make sure your pipe is clean, and rotate your pipes...give them time to rest.
What are you smoking? Virginias are prone to create a little more tongue heat than some others. And, some aromatics can goop up your pipe so much that they can burn hot.
Of course, there's always the possibility that, even though you don't think you're puffing too hard, you might be. Some tobaccos behave better when you "sip" them slowly.
Mike