Well I've never been pregnant and once again I stress that I'm no expert with symptoms, but let me tell you it sounds a lot like what's happened to me. The cause? Nothing
I tell people that I'm already getting old (at 19, lol). Apparently it happened to my mother, too. She used to be a really sharp person, like I once was, but by the time she hit 30 she was practically senile (not medically, of course).
So yeah I think you're just changing a bit. And until it gets so bad that you can't remember how to get home, you don't have anything to worry about.