Period came 2 weeks early!?

SJ
Why is it that when I wasn't trying to get pregnant, my period came every 28 days and now that I am it's all messed up? Last month cycle was 27 days and this month it was only 15!! Wasn't scheduled to start until 12/2 or 3 and it came today (Nov 21). Any insights? When would I have ovulated? Probably not when I was trying (this past week).