First time using plan b

I had unprotected sex Friday the 13th but it didn't last long because my boyfriend didn't either 😜 I'm 90% sure he didn't cum in me and I got off of him in time but he got me plan b just in case. I only got a little headache when I took it less than an hour after having sex but I think it was just a stress headache. 
I'm 20 and over the weight "limit" for plan b and I don't use birth control other than condoms but starting yesterday (Wednesday) I got really bad cramps and spotting and today it turned into brown/really dark red blood but I'm still in a lot of pain like a period but the blood isn't as much or coming out like a period (if that makes sense?) 
I know plan b is said to cause "periods" or make them come early and my actual period is due in 2 weeks but I just want this pain to end! 
Is this normal? Any advice?