What Causes Spotting In Between Menstrual Cycle?

Hello, I am a little concerned about the fact I am experiencing spotting in between menses. This is the first time this has happened to me. I've been on oral contraceptives for over a year and have always been very good about taking them on time. I have however missed one pill about 7 days ago, but I have not been sexually active for over 3 weeks until 2 days after the missed pill. I made sure to take my pills on time the days following the missed pill and should have been protected. The spotting I am experiencing has lasted for 4 days so far. It began the day after I had intercourse this means there was no time for the implantation process which is usually 6 - 12 days after conception. Could you give me an idea as to if I have an infection, or possible pregnancy. I haven't had any pregnancy symptoms and it seems unlikely. Thanks.