For a due date of Sept 15 conception is likely to have occured around dec 23.
This means sex in the 5 days prior to and including dec 23 is what is likely to have resulted in this pregnancy.
This is accuarate based on the estimated due which is calculated from the u/s scans which measure the baby's growth and are reasonably accurate.
So it looks like the greatest probability is that you are the father, however the only way to know 100% is paternity testing.
Is it possible that she was sleeping with the other guy more than the one occassion she mentions?
moral: don't sleep with your ex.
