cd18 clots- implantation or ovulation?

Ali
Hi everyone, hoping someone can shed some light on this. Have been TTC #1 for 11 months now. Anyway on cd18 I began bleeding for a few hours. It was dark brown blood, and was coming out in clots when I wiped and when I went pee. Also had slight cramping. I might add this bleeding also started during strenuous exercise (basketball game). A few hours before I started bleeding I did notice some ewcm
I do not know when I ovulated As I didn't have the energy for opks this month. 😓 I usually ovulate anywhere from cd12-cd17. Period is due in ten days (28-29 day cycle) 
So my question is could this be implantation bleeding, ovulation bleeding or something else/more serious? If it is in fact ovulation spotting, is it typical for that to be clotty? 
Thanks in advance!