why would my boyfriend react like this to a joke?

[ChinaRose1]

hi recently me & my bf moved into a house together, yesterday there was a little sock in the laundry basket & he said was it mine (but i no it was his little nieces because i saw her with them on) and i said 'no, one of the girls youve had round must of left it' as a joke but it probably came out more serious than i expected , but his reaction bothered me i thought he would be like 'oh yeah one of the millions' but instead he said 'what?! what are you talking about you weirdo!? no girls been here, noone nos were i live' hes normally jokey but this bothered me for some reason but i dont no if hes trying to hide something or he couldnt tell i was joking!? help plsss its bothering me!! thanksss