What Causes Light Vaginal Bleeding Post Unprotected Sex?

Hi i had unprotected sex on January 2,2015 and on the 4th I started light bleeding from very faint pinkish color and then from dark brown to light red then back to faint pink blood. It lasted for a whole week. Now it stopped.I have irregular periods and I had an abortion in August 2014 and my period never got regulated after.so my question is.. What does this light bleeding mean it never happened to me before? Should I be worried? Also I'm not sure if my partner had ejactulated in me. I asked him if he did but he shrugged it off and now I'm not sure so please help me out thank you!