if you got your period in the beginning of October, then having sex on October 14th would be the perfect time to have gotten pregnant. Sounds like it's definitely your partners. You would have been 39wks pregnant when you delivered as well, as compared to 40 weeks if it was to the other guy.

luckily, it's not expensive or complicated to do a paternity test after the baby's born. you can even get kits at walgreen's... if you really want to be sure, that is.