Is this cheating?

So I went out for my birthday and me and my boyfriend argued and ended things. Then I went back to a boys house with a group of friends and slept with him. I've felt guilty ever since but is this cheating if we finished before I did it? We got back the next day but I told him a little bit of what happened but not all of it and he said just forget about it. But I can't because I've got into my head that I've cheated. But how is it cheating if we ended before I did it? And I had no intentions of doing it I did a load of drugs (for the first time) and drank loads of alcohol so I was over confident and did the drugs. Someone please tell me and put my mind straight.