HELP me FIGURE this out. AM I THE FATHER?

Alright, I'm not sure if i am the father. Basically my girlfriend still had feelings for her ex and cheated on me. she is now almost 6 months pregnant with a baby girl. she says she is pretty sure i am the father. ON december 13 2010, we got a sonogram and it was 6 weeks and 4 days. that means that she got pregnant around october 27 2010. Around November 3 couple days after halloween, i went to a party to where i saw her ex... and i asked him when was the last time they had ...(i had no idea she was pregnant i was trying to find out if she lied to me in wich she did lol) he said to me abt " 3-4 weeks ago" and this was said on november 3rd. NOw... (she says it was arounf the beginnng of october, but i belive him more. She got her period around the 26-27 of october or her Last period was around then .... now even if she had 2 weeks bf November 3rd, instead of 3-4weeks is it still my baby? and how did she get pregnant almost on her period? ( but one more thing couple years ago she found out she was pregnant of 2 months... but the first month she got her period.. but got an abortion...could it be that it happened again and maybe her ex is the father? im so worried... but all the calculation go to me as the father ....unless there are other chances that im not aware abt. PLEASE SOMEONE HELP ME... Ive been there for her since day one... my mom is trowing her baby shower... i dont wanna do all of this and then in the end it's not even mine. PS the dr said the due date is AUGUST 3rd 2011.