What Causes Short Periods With Bloating Of Stomach?

hello doctor sahab, i had sex with my BF in nov.and then i have take ipiil and also got my periods on the 15th but period was like for 2 days only then on the next month i got my period for 3 day it was not like i m getting as normal for 4 days full.then in january i got my periods on 25th but for 3 days again and my stomach is swelling as if im getting pregnant i have also done pregnancy test 2wice and its showing negitive however im afraid if im pregnanat..pllzzz answer me as im veryy afraid sir,,,thankuuu very much sir...