I had sex a week prior to my period and took EC pills immediately. I took a blood test the following week and it detected negative. On a day after my period, I woke up with a full pad and the blood was red with little clots and I washed up and changed so the second was almost filled up too. On the second day the flow was quite scanty with cramps. Today is the third day, my pad was filled and I saw clots and normal period tissue discharges, but there is no blood flow tonight. I am having a slight headache although I had a stressful day, but I am scared and worried. Could it be implantation bleeding?