Boyfriend back handed my arm in an argument

El

So apparently while I was working or home or somewhere my phone dialed my boyfriend left a two second recording.. recording was oh no u.. laughing in background liz.. that's all and my boyfriend is accusing me of flirting and wants an explanation on who I was talking to meanwhile I do not remember this conversation at all I work and talk to cust all day long so I don't know who the fuck I was talking to .. I'm faithful 100 percent.. I know by me laughing I didn't do wrong so wtf!! And I might be pregnant with number two so idk what the heck to do is backhanding my arm a start of physical abuse?