What Causes A Light Menstrual Period?

I fear i maybe pregnant. I havent missed a period, I had sex the first week in august and had my period the second week right after, but my second period was lighter but i havent had sex since that first week in august. Also about 2 and half weeks ago i had an ovarian cyst rupture and they took a pregnancy test for me and said I was pregnant and had no UTI but I still have a fear that i am. I have no sickness, no frequent urination, no cravings, nothing but still. I'm scared that i am.