What to do!

Can anybody help, my last period was 21st September, we had sex on the 30th September, 2nd October and again on the 5th October! My period was due yesterday (19th oct) had cramping for about a week before but nothing, still no period today but brown colour on wiping this morning! Still getting slightly cramping but have no other symptoms! I took a pregnancy test yesterday but negative! I am confused, my period is never late and also bang on 28 days!