What Causes Brown Spotting In Underwear?

Doctor, I am 20 yr old female.I get my periods every month but my periods last for 5 days.WIth bleedin heavily on 1st and 2nd day and negligible on 3,4,5.Though i get my period everymonth,it keeps gettign preponed and postponed.i got my period on 2/4/11 and next i got my periods on 14/5/11. on 14/5/11 i had unprotected sex with my boyfriend.on the first day of my periods.but he DID NOT ejaculate inside me.But obviously there was his precum.And i am gettign tensed..are there any chances of pregnancy?!?!Also on 25/5/11 and 26/5/11(today) i observed some brown spots on my underwear..what could this be?i am very tensed! please revert back soon!