What Causes Brown Vaginal Bleeding Along With Intermittent Pain In Abdomen?

My last period is on 28/11. My cycle is usually 39 to 40 days. I got an unprotect sex on17/12 to 22/12. I got brown and pink discharge starting on 28/12. It last for 4 days, it doeant look like a normal period. I feel pain on and off at my left lower abdominal, no sore breast as usual before period. Nausea and bloating. I test the urine hcg on 5/1, but it show negative. Am i pregnant or just something not right? Helpp..