Relationship trust issues

I'm having trust issues with my other half. Tuesday evening/ night I left him and a female friend at mine while I went out for a few hours they both had some drinks. While I was out I had a feeling that something wasn't right twice I rang and took him ages to answer. When I got home I had a drink with them n he told me that they had done stuff oral and things no actual intercourse at the time I didn't react because I'd had a drink n was calm ish the following day I was the same I just couldn't bring myself to talk about it. But when I did he said that they were just messing about n it just happened she says she doesn't remember a single thing which I think is complete and utter rubbish. I feel so low n every time I mention it to him his reply is "oh, this again" like I'm making a big deal out of nothing. Today I dropped him at his mums n I can't take my mind off what happened on Tuesday. Please help I really need some advice thanks x UPDATE : Thanks to everyone that commented I'm so grateful. She's admitted that it was her doing and also her intention. She was my partners friend not mine. Her sister is my best friend she asked me not to tell her as it would cause her problems but I seem to have accidentally forwarded the whole conversation to her sister oops 😂 I know I was stupid leaving them alone but I trusted him he's never done anything like this before. It's not like him. But he will never do it to me again! X