Had protected sex then, 2 days later got my normal period, a week later i start bleeding light pink stringy blood. what is this?

Probably normal: This might be a normal response to recent sexual activity. Never hurts to get a home pregnancy test but without pain, and as long as it is light and goes away quickly it is likely a normal situation.

Many possibilities: The website given below lists the causes of inter-menstrual bleeding. The timing of your bleeding is consistent with ovulation bleeding. http://www.patient.co.uk/doctor/intermenstrual-and-post-coital-bleeding.htm.