Happened to me once. I think it was because the day was really long and tiring. I met this girl and we had a long date that started at like 10AM and then around 9PM we were back to my place and I just thought "Can't think of nothing else to do so might as well try to tap that". I went for it and she was into it. I was hard at first but couldn't keep it up like normal. And honestly it felt a bit weird after but overall I was basically like ¯\_(ツ_/¯