What Causes Delays In Menstruation?

hi i have taken birth control pills in 7 years, and i have always known when my period is going to start, and it has been right on time everytime. 2 months ago i was 4 days late, and when i started to bleed, it only lasted for 1 day. the next period i bleeded for 8 days straight. is something wrong with me? i havent got waightloss or anything no stress... am i sick? thanks girl from DK