What Causes Brown Color Spotting During Menstruation?

i have a huge question that has me really worried...i got my period on the 22nd of august and got off it on the the 26th everything went as usual the month prior i received it on the 24 and ended on the 29th so all has been normal until yesterday when i noticed i was spotting a dark brownish color...so i tried to not think of much and today when i woke up i was bleeding a darker red than usual and very very watery......im not in any kind of birth control or any stress....i did have unprotected sex but my partner didnt cum inside me...please do you have any answers to what might be wrong with....