I was in a relationship for about 2 months. We always used protection except one instance (10 October 2009) when we had unprotected sex for about 5 minutes before i came to my senses and put on a condom long before ejaculation(never ejaculated inside her).
I first slept with her on 04 September 2009 (with condoms). Now she says she's pregnant with my baby and delivery date is towards the end of May 2010.
She first said she slept with her ex on 29 August 2009 and when she saw my concern changed the story and said there was never another guy, she had lied to protect me. However, 29 August 2009 ties up with the EDD of end May 2010. Moreover, i always used condoms, except that 5 minute incident (10 October 2009) where there could have been pre-ejaculation sperm but still, due date based on that conception date would be 03 July 2010.
Is there something i'm missing and could i be the father?
There's very likely a lot you're missing. You're relying on information from a woman who's told you she deceived you about another possible father. The difference between August 29th and September 4th is way within the margin of error of EDD. It could easily be either of you. If you've made some effort of using protection the chances are lower that you're the father but certainly not impossible.
More importantly your Ex cannot say with any certainty that you are the father. If you're reasonably sure that you're not the father tell her that you're not willing to assume responsibility for the child until paternity is proven through a DNA test.
But i used protection on all other occasions except for that brief moment on 10 October 2009 (i never ejaculated inside her), when we had unprotected sex for about 5 mins and then i put on a condom (long before ejaculation) the rest of the time. Based on that encounter the delivery date would be 03 July 2010 which is approx. 4-6 weeks off her EDD of towards end of May 2010. The May 2010 EDD ties up with the conception date of 29 August when she was with the other guy (although she changed and said there is no other guy).
You had sex with her, unprotected sex on one occasion. There is the possibility, no matter how small, that you are the father of that child. Estimated Date of Delivery is an estimate, its exacting, but not infallible. A paternity test will determine with certainty if you are or are not the father and it will settle the issue legally for you.