It's probably just bunked up, when our sink pipe blocked up I was poking all sorts of twigs up there to clean it out, turns out that the scale that had built up meant that the inside of the pipe resembled a tube of concrete

So I has to change eveything over to make it nice and new again. Stick some si k unblocked down it and see if it clears the smell, leave it o ernight to do it's work but afterwards pop the trap and see if it's full of black sludge, might just need to clean it out properly.