Why aren't I falling asleep yet?

I took some percogesic earlier. I took two tablets, so that's 650 mg of acetaminophen and 25 mg of diphenhydramine. Usually antihistamines make me fall asleep really fast, why isn't working? I never take them just as sleep-aid, it's always for something else, but then I get drowsy. I'm almost just like wide awake now, not tired at all.