Suggestions please...

So we're having a bit of a dilemma here at the Stiffie household. It's Mady. No, there's nothing wrong with her, it's just something that's happening. Her diaper is leaking at night. Not just a little leak either, it's like she isn't even wearing a diaper! It all started about a week ago when she started sleeping on her tummy. Now every morning i have to change her sheets and wash her off in the front. I know it's not because her diapers are too small, because she is in the size 2's and they are 12-18lbs, and she is only 14.5lbs. We even tried the Luvs (aka, "boy diapers"), and no luck. I'm thinking my only options are to either double diaper her at night (making her look like a snowman), or to find a different type of diaper? Any suggestions?