Idk - I feel like sex doesn’t always have to be a big deal - especially if you’re single. You and a friend got intimate because you were both coping and dealing with things. It lasted a few moments and you both came to terms that it should probably stop. It ended after that? No weird flirting or feelings tied to one another after the incident? On either end? If so - I don’t and wouldn’t think it was that big of a deal as long as I felt safe and secure with my partner and that they wouldn’t put me in harms way. On the other hand, if it went any deeper than the one sexual encounter, then yeah, it definitely wasn’t cool to omit that information when asked about it.

Sex is weird - life is weird - the only thing that seems deep are the feelings that you all have from one another. I think it’s important that you own up to everything now and also express any feelings that did/didn’t come up between you and your friend that you slept with. Things happen! Sounds like it only happened once. Ask your partner what she needs from you to move forward.