Do you think i could be pregnant?

i'm 21, i weigh 114 and i take yaz birth control always on time. my boyfriend and i are safe about . recently i have been nauseous and nervous about all the work i had to do and school from having so many snow storms come through my area and consequently missing a lot of days. when it was time for my monthly cycle, it was hardly even there...about 2 days it was completely gone so really the actual part lasted a day. could this be due to my weight and stress? i did take a pregnancy test the day i started and it showed up negative but i guess i am constantly worried about everything. any advice you may have would be greatly appreciated. thanks!