Next day is a little early to get symptoms of anything that could cause a rash (herpes is the big one) that takes at least 3 days and oral is a slim chance, I would assume it's bacterial, or maybe caused by too much attention from her to the tip of the penis. I've woken up in the morning after a night of "rough" oral to marks on my penile tip before, give it a week, or two and see what it does. If it's not gone, or you get any itching burning or discharge go see the doc
Btw I meant " aren't Tall "