Early periods/trying to conceive

My period has always been pretty reliable on coming around the same time every month. But two months ago it came about four days earlier. I started charting my period on here and last month it came twenty days after my last period  and now it just started again. That makes it only 16 days since my last period. I know it's supposed to be about 28 days between periods. It worries me a little bit because it seems to be coming three or four days earlier than the month before. I also haven't been on bc for almost a year now. Anyone have any ideas why this might be happening? Or is it just normal and something I'm not used to?