I wouldn't worry too much - when they say you are more fertile after having a baby - they mean like right after - like the first 3 months. And since pre-ejaculation has so few spermies, chances are if there were any at all, they died off. My dh and I start off without using anything pretty often and have never had a problem getting pg - and I'm very fertile normally! All our babies have been planned and I know my body well enough to know when I'm ovulating and stuff, too.
I mean, certainly anything is possible, but the chances are so very slim that you have gotten pregnant this time.

Rest easy!