Help please

This morning my boyfriend ( baby father ) turned his phone on whilst sitting next to me and a notification came through with a Facebook girl wanting to be friends, this girl was from the same town as us and when I asked him he said he can’t get no messages or anything from anyone on Facebook, he said he only uses it to sell cars and buy cars, what do I do I’m fuming, makes me wonder if he’s been talking to this girl I literally feel sick

Do you think he’s been talking to her and been hiding his phone turning it off on night time

I’m looking to end the relationship over this how dare he lie to me