heavy blood 12 days after period?

Rebecca
Usually have a very set cycle.
My period finished 12 days ago, for two days I have cramps and a sore back and now I'm bleeding heavier than I usually do on my period? 
I took ovulation test, and have a surge so I thought I was ovulating but I got my period instead? 
Is this weird? Can someone please shed some light as to why this is happening