What Causes Brown And Scanty Menstrual Period?

Hi, i'm a 25yo lady. I had my previous menses on the 2nd of may, and it lasted for 2days. I had unprotected sex with my fiancee on the 11th 14th&15. my cycle is usually 28day, but now a day has added to it makin 29days but the blood is brown and scanty. Monthly Breast tenderness has stoped, and I feel a little cramp and diarrhoae. Please tell me what this means.