Can i be pregnant andd not know it?

me and my bf have been trying to conceive since october my period has never been late i usually start between the 20th - 25th of the month my period is five days late this month and i have been nausea tired moody and increasingly hungry. today i started what i think is my period but its really light. i have also had some low abdominal and back pain. and a few years ago they found cyst on my left ovary. so i was wondering if i could be pregnant please help p