ummm confused??

Me and my fwb of a long time now had sex last week and the condom broke. The next morning we went and got a plan b and took it within 8hours... she told me my period would be weird, but I didn't really expect much to change ( sex doesn't effect my period days, and it's always very vey constant no cramps EVER! I love my period). This is the first time I've ever taken a plan b, so I figured weird would be a little spotting here or there, so my period just started, 10 days before my normal period should've started! And i really sympathize ladies with cramps because holy balls I have never experienced anything sooo painful before and I've had my period for 4 years now!! So I just went to the bathroom and it was like a glob of super dark red looked like brown stuff came out!! I have never experienced this so I guess I'm asking is this normal? What is going on? Why would it be soo thick and dark like that, shouldn't it just be a normal period?!?! Let alone 10 days! Early.