It may not be the tobacco. If this is a new pipe that you are having a reaction to, then it may be something with the pipe. If the pipe has been stained and you are in the break-in process, this could be causing the issue. Not so much what you are smoking, but more of what you are swallowing. If the stain is burning off, and you are mixing the residue with moisture, which is a natural occurrence when you are smoking, then you may be inadvertently swallowing said residue as it exits your pipe stem while you are puffing.