It sounds like it's definatly possible you could be the father. When I thought my baby was conceived on January 25th, I had an EDD of October 16th, so the dates are pretty close.
Yes it is possible. Keep in mind that early ultrasounds can be off by a couple of days so the discrepancy between what she said is her lmp is accurate. The only way to know for sure will be to have a paternity test done after the child is born or through an amniocentesis.
Every time you have unprotected sex there is a chance of pregnancy. It really depends on where in her cycle she ovulated. If she has only had relations with you than it is your baby. The best way to tell would be to request a paternity test once the baby is born to confirm.
Good Luck
U could be. Did she have sex with anyone else