It could be possible because if my prenatal paternity test is correct...i got pregnant by no ejaculation! Crazy. Precum got me preggo. :D
Do you know that it a doctor tells you that you are 6 weeks 1 day GA (gestational age), he or she is counting from the first day of your last period? (Or from a computed last period if you are irregular?) All medica counts of pregnancy begin at that point due to it being something that can be seen, versus ovulation which is hidden and unseen. If a doc told you on November 5 that you were at 6w1d, you got pregnant about 4 weeks 1 day prior to November 5, or around October 7. The dad would be your ex-now-boyfriend. Not the guy from September 19.
Incidentally, if the guy from September makes a stink, just tell him you will see him when the time comes for a DNA test. You have nothing to worry about. Explain how doctors count pregnancy (if you get one of the "Your Pregnancy Week by Week" or "A Child is Born" books you will find the weeks count explained and can show him). But if he doesn't believe you or understand you, be prepared for him to blab to your boyfriend, and this WILL make your boyfriend nervous. So tell your boyfriend soon that you must have conceived around October 7, and have the doctor explain how pregnancy is counted by medical people so the boyfriend will understand that "6 weeks pregnant" in doctor language means 4 weeks since conception.