What Causes Vaginal Spotting With No Regular Period?

I am a day late on my period. Today I have started some light light light bleeding. I have all my pre period symptoms normal. 3 weeks ago I was intimate with a guy but he never penetrted. He did rub the end of his penis on the very top of my vagina around the clit. Ive just started back school so im adjusting to my school schedule. Could I be pregnant