WTF! Fucked by the wrong guy!!!

How cheeky can a flatmate be? Have you ever brought your girlfriend home, blindfolded her and then disappeared from the room for a moment to get a pair of toys? This exact case happened today, only my boyfriend's flatmate took advantage of my helpless situation! What's the best way to explain the strange cum to my flatmate? Oops, sorry your flatmate fucked me while you were away and I didn't notice?