Where did it come from?

I'm in the pediatrician's office this morning, nursing M, while K and S have their 3 and 5 year old checkups. I finish feeding M and put her back in the carseat. Soon thereafter I notice a wet spot on my shirt, near my waist. "Who leaked on me?!" I exclaim. "Or did I leak on myself [breastmilk]?" I mutter. The doctor looks slightly alarmed (note: pediatricians without kids themselves take everything a little too seriously, especially mysterious bruises on 3 year olds). Anyway, I discretely lift my shirt and give a sniff, hoping to smell breastmilk. Nope. It's pee. Gross. I pick M back up, and give her clothing a feel. It's completely dry, and she has a onesie shirt on with pants overtop. There's no way pee could leak out without getting her clothing wet. So where did it come from? Makes no sense to me.