Broke my trust

A while back, I saw a new girl that followed my boyfriend on Instagram which he followed her back too, she had that sort of picture of her In a bikini which made me sort of think "who is this girl" "does he know her?" Etc etc.. but I kept it schtum about it as following was kind of harmless in a way. Anyways a month later or so it's peculiar how I found out she's sent a message request to talk to me via insta direct message saying "hi hun can I speak to you quickly?" My heart started pounding briefly only because I know this was something to do with my boyfriend as he was the only one that followed her and no other mutual requests! They both didn't have each other as friends any more either which was weird enough, so I replied back saying "hi yes sure" I appeared blunt for a reason as I knew I didn't like this person already! She didn't reply back after 2 days which was odd too. Anyways that day when she messaged me I didn't want to tell my partner I had a strange girl messaging me incase he brings up lame excuses I wanted to find out from her first but because of the long reply I asked him who she was face to face and said I want explanations and his reply was this "this girl messaged me last week sending me a picture randomly of her pussy, I swear to god I do not know her and she done it whilst I was at work whilst I was with my colleagues" so I said "what the fuck you let me think nothing has happened and some trashy girl sends you a picture last week and you didn't tell me? Show me the conversation then on your phone" he said he deleted it and didn't have it which makes things a lot more suspicious as if there was something he probably didn't want me to see? He says "I'm sorry I should of told you but I don't have the convo, my work colleagues took my phone and joked around by sending funny stuff" I was like "so you let your colleagues humiliate me when they know you have a girlfriend by telling her she has a nice pussy? I find it funny how you removed evidence" don't get me wrong her profile was clearly a fake profile but I was so angry by everything I didn't know what to think!!!! Anyways I was still awaiting for her reply. 2 days later she messages me at like 1am in the morning showing me a screen shot of part of their convo and I was asleep, my partner went on my phone pretending it was me (I was not aware) replying back to her saying she's a liar show proof of the whole conversation but she wouldn't bla bla bla. I woke up in the morning and he told me he messaged her on my phone which I was pissed off because he could of atleast woke me up so I could speak to her myself? He was like "she won't show the conversation where I said to her that I'm not looking to cheat" I was like that means many things then doesn't it? You can tell a girl "you have a nice pussy but I'm not looking to cheat" but funny how you removed your conversation with her isn't it? He has apologised to me and he admitted he was in the wrong for not showing me in the first place but he's broken my trust and I don't know what else to think?