Laura,

It COULD be your litterbox. Especially if you've been using it for a while. I've started replacing all my litterboxes because I've had them a long time. After a while the plastic absorbs the odor and no matter how much you scrub, disinfect and clean them, the odor stays. Pet Supplies Plus has a sale on jumbo hooded litterboxes for $15.00 for the month of May. It might be worth it to replace it.

Ally Cat's Mommy is right, it could also be the food you're feeding them.