My girlfriend cheated on me and had sex with her ex once last Dec. 27, 2013. She has irregular menstruation and we thought that she could never be pregnant. We had sex on Jan. 1, 2014 and has been doing it since then. Last Feb. 19, 2014 we learned that she was pregnant. In her ultrasound on the 20th, it was six weeks old. Am I the father of the child?