What Can Cause Dark Brown Spotting?

I'm spotting and I rarely do this but I've been exercising for the past week and it's strenuous. I heard it normal but it's been getting worse, I know I shouldve stopped after I started spotting but I never thought it was because of the exercising, My question would be why, and why is it turning somewhat pinkish red and then turned almost dark dark brown this morning? I'm not suppose to get my period for another 9 days. whats going on? yes I have had unprotected sex.