Super early AF or...?

My cycles have always been SUPER regular, 30 days, no mid cycle spotting or anything. AF isn't due until the 15th, but I started bleeding yesterday morning out of nowhere, with no telltale PMS symptoms, and the blood was medium-heavyish, red with some small clots. It's all but stopped today - just a little spotting. I'm not going to test until my original AF date, but could that have been IB instead of a waaaay early period? Anyone ever had this happen to them?