Pregnant but who's the father

Hi I'm a male and my girlfriend is pregnant, we had been on and off seeing each other and just got back together properly a bit over a month ago ..   she realised she was pregnant just after we got back together..   We straight away assumed it was mine as we had unprotected sex a few times from the 19th - 22nd of july period  but then i also found out that she had had sex with another man 18days after me she said they used a condom and was fairly sure it didn't break or anything that was on the 9th of august ..  We got an ultra sound on the 2nd of october and they said their estimation was that it was 9 weeks and 4days …now that would be the date four days after i had last had sex with her , so we originally thought that that was possible as sperm can live for a few days ..  but then as i looked into online more i realised that they say the date from when they think your last period was , so then when your ovulating is about two weeks after that so that would bring us to pretty much the exact day that she had sex with the other man..       She says that her last proper period was on around the 5th july she thinks (but is not certain)   but then she says that  she bleed a little bit again on the 26th july four days after i last slept with her but she says she only had a bit of blood for one day and that her period is always usually four days long ,   so we originally assumed that this was when the egg was fertilised as we read that it is possible to get a little bleed from that ,  but now that the ultra sound dates are telling us different maybe that was just her real period , but its just really strange …  basically i want to know is there any chance that it is mine ??? can the ultra sound estimate be off by this much??Sorry it is so long but i didn't want to miss out any details thanks