Sex with someone else during a break up

Me and my partner separated and I stupidly had a one night stand that I instantly regretted. I haven’t said anything to him. But I feel like I have a sti. As far as he’s concerned everything’s fine in the bedroom. But I smell a fishy odour that I want to mention but I’m almost certain its coming from me. Me and him speak about everything and things like “smells” get spoken about it they occur and we dont have shame when it comes to one another lol. But I don’t know what to do. He and I are back together and I have just been to get tested but are yet to receive the results. If I have something how do I tell him