What Could Be Reason For Irregular Periods?

Hello,

I was due for my period about the 18th or 19th of this month. I took an at home pregnancy test on the 20th to check my status and I recived a negative. Immediatly after this I got the light brown mucusy spotting which signals my period normally. For the next 21/2 to 3 days I had I would say a moderate flow, but the blood was more like heavy spotting as it was dark brown/ black at times and darkbrown/red. It was moderate for a day or two(enough to fill a tampon and 2 pads, and then tappered off. I have not gotten anything since I would say the 25th of this month. Today my breasts started to get sore and I feel a bit crampy and fatigued. I have not had unprotected sex once, and am always super careful. I would like to mention though, that my period last month was 4 - 5 days late, so I have been a bit irregular. I took a pregnancy test on the 26th in the morning and this was a definite negative. What could this be?