What Causes Prolonged Bleeding With Brown Discharge?

I started my my period on Feb. 4th and it was mostly the brown mixed with normal discharge until the next morning. I have a 30 day cycle that always starts on the 6th of the month so this was a little early. Then I bled pretty heavy, which is normal for me, for about 3 days and then it turned into the brownish stuff again for the most part. Yesterday the brown was subsiding so I thought my period was over because that's what it usually does...Then last night I felt a cramp that felt just like it normally does on the first few days of my period and went to the bathroom to check it out. I sat down and went pee then wiped and realized it was bright red, so I stood up to look in the toilet and there was blood in it and then it started running down my leg. I sat back down and blood literally poured out of me for less than 1 minute, then it slowed to where there would just be red if I wiped. This was different then a normal period, the blood was BRIGHT red and thin, almost watery. I decided since I didn't have any pain aside from the one cramp to put a tampon in and go to bed. When I woke up this morning I took the tampon out and it was mostly brown....what's going on? I normally bleed for 6-7 days, I have NEVER bled more days than that. I have also never had a period start and stop at the end of it, it's always in the middle. I had unprotected sex about 2 days before my period started, is it possible I'm pregnant?