Possibilities of pregnancy on the pill ?

Hi I am 18 years old and take the birth control pill, however last month my period/ break through bleed only lasted barely 1.5 days rather than its usual 4 and it started on the last day of the 7 day break. I am now nearly due to have my period again and take my last pill tomorrow. I have had nausea a couple of mornings, constipation then diahrrea (sorry) and have backache, cramps and a LOT of discharge. I have also gained 4lbs I dont know whether i am pregnant or have just thought about it too much. took a test after the last period which said negative but its been a strange month and i dont really want to keep taking tests but cannot stand the wait. any advice? anyone had the same thing?? p.s. this month my pill has been taken in the morning some days and the evening others due to busy lifestyle . .. help!!!