Why does my dog pee inside?

My boyfriend and I adopted a three year old min-pin yesterday. He's housebroken and he had no accidents during the day. We walked him for the last time around ten at night and he woke us up around six a.m. the next morning wanting to go outside. My boyfriend walked him then came back to bed. We woke up two hours later to find that he had heavily soiled not one but two different spots in the house, what could cause this behavior?