+1 on getting it checked out. Are you certain that the bleeding is vaginal and not coming from your bowels? If it's coming from your bowels, seeing your PCP and/or a GI person would be more appropriate than OB-Gyn.

Having said that, it's not that unusual to have bowel irritation related to running so try not to worry. The chances of it being something serious are small but the potential risk if something serious is missed is high and with serious consequences so please follow through with the MD appointment.