late and unusual period... i'm scared!

i'm really confused and scared right now. for the last 6-7 years, ever since i had my period, it's always been very regual. it always last 5-6 days, nothing shorter and nothing less, a 24-28 day cycle and i normally bleed through 3-4 tampons a day but my last period was late it took about 31 days to get and when i did get it it was very light the first day, second day it was normal and then for the next couple days it was very very light bleeding... i didn't even were a tampon or a pad! and it was a brownish color. and the week before i went to bed every night with headache and i still do. and when i did get my period i felt the normal cramping. my boobs hurt... they hurt right now but early today i took a pregnany test and it said not pregnant... i'm scared! is there something wrong!