I notice that after heavy cable crossovers. Flyes may have the same effect; what you're feeling is probably trauma caused by working the muscle at an extreme stretch. At the bottom of the fly motion, if you go below parallel, you're really stretching out your pec. Exerting it then isn't necessarily bad, but it would probably cause more microtrauma than if you didn't descend quite so far. Nothing to worry about, unless it's taking too long to recover.