What Causes Delayed Periods With Heavy Periods During The Previous Cycle?

Hi, last month I had a very unusual short 3 day heavy period, with clots. This month my period has not shown up. I am almost 2 wks late. I took a pregnancy test but it came back negative. I am so confused BC I have never missed a period without being pregnant. I don't feel any cramps or anything indicating that's it will be here soon...plz help? Ty in advance.