help please!

So today was supposed to be the last day of my fertile window. Last night I experienced bad cramps in my lower stomach. Almost like period cramps but my period isn't due for another 12 days.. Today I still had light cramping and went to the bathroom to notice I was bleeding. Not like a lot but not a little either. Does anyone have any explanation of what this could be? Also I don't know if this could have something to do with it but my last period lasted longer than usual. Thanks in advance!!