What Causes Light Brown Periods After Unprotected Sex?

i had unprotected sex 4 days before my period started but my period which is uasually very heavy was very light and brown and it lasted for about 8 days. now i have this tight feeling in my lower abdomen it feels tighter when i cough and breath heavy. a little lower back pain as well. could i be pregnant even tho i had a period and the test said negative...???? help im so scared plz