What do I do lol

My bf & I went to the store not too long ago, a guy came into the store shortly after us... I didn’t look at the guy or know who the guy was. Anyway, once we get in the car he goes “so that’s the guy you fucked? What a downgrade” I was confused, I told him “what? What guy?” & he says “the guy in the store” & I said “I don’t even know who that guy was, I didn’t look at him. Who was it?” He says “I don’t know” & I said “how can you accuse me of sleeping with that guy if you don’t even know who he was?” Anyway, now he’s mad downstairs & idk what to do. I haven’t slept with anyone else in months, I hooked up with someone while my BF & I were split for a month & a half.