Cheating or Assault?

I was peer pressured into drinking and convinced to stay at the party I was attending instead of driving home to avoid a DUI. I don’t remember everything but I feel like I was coerced into having sex after I laid down. I didn’t trust that I’d be able to leave without worse things happening and I feel horrible. All I wanted was to be in my boyfriends arms when it was happening. I fell asleep for a few hours after, woke up and felt sick to my stomach to where I just left as fast as possible without making anything worse. I want to forget it ever happened and lock it away with past traumas/memories. I would never consciously cheat and never have before, and I never want to put myself in that sort of situation again. Is this considered cheating or being assaulted? I don’t know what to do. I want to be with my boyfriend for life, and I know he feels the same way but I’m terrified of telling him about this because I’m afraid he’ll leave me :(