your last period and your due date don't match - there's like a 2 week discrepancy there.
do (did) you have regular periods or not?
based on your lmp then your likely conception would have been jan 3 and your due date would be sept 26 (meaning mostly likely your ex's)
based on your due date (is this ultrasound confirmed?) your likely conception would have been jan 15. (meaning most likely the other guy's)
the only way to know for sure is to have a paternity test done after the baby is born.
I've moved this to the appropriate forum.
Please do not type in all caps.