Need to be sure of father

I had my period on September 2nd roughly- I slept with my exboyfriend on the 3rd after midnight. I then slept with my current boyfriend anywhere between the week of the 12-17th. I found out i was pregnant on September 24th and went for the unltrasound appt on September 27th.. Nothing was able to be detected on the sonogram. I am unsure on who is the father and i need to know for sure before i carry.