My period has been irregular for the past 3 months. Last month, my last period, began the 21st and lasted over a week which was really strange as mine is usually 4-5 days. I don't believe I could be pregnant as my only encounter this month wasn't full intercourse it was for like 3 min. I have been cramping in a strange on/off pattern for 3-4 days already and am sure I'm late and I just discharged a brown-like color.

Brown is "old blood" probably indicating the start or end of a period. Sometimes cycles can be off for various reasons such as stress, illness, or simply no real reason at all. If you're late, its time to take a hpt.
Oh, and I'm not sure what you mean but "full" intercourse but any time there is penetration (no matter how much time) there is a chance of pregnancy or STD.