I just remember happening this to me once. I was talking to this British girl and after a couple of minutes I took over the accent - or at least I unconsciously did. It must've sounded horrible, haha!
Only thing that I do notice on a more regular basis, is that my accent gets thicker when I am around my dad for instance. His accent is thicker than mine. When we meet, we both go nuts on it.
Very interesting read! I don't think you should worry about it too much :)
How wild! Did she notice it or did you?
Did you talk like your dad more when growing up, then transition out of it when being around other people? I've heard some stories like that before, especially when people go off to college. It's easy to go back to your roots though. :)