Have you been finding less shares?

Your payout may change based on the amount of shares you find, and the variation of the pools hashrate.  If the pool hashrate goes down, the pools [share] difficulty goes down and you will earn a little more due to finding more shares.  If the pools hashrate increases, you will earn less.  The other side of this is as the pools hashrate increases, more blocks should be found; and as the pools hashrate decreases less blocks would be found. 

So your payout may change based on the pools hashrate, if your hashrate stays the same.