We felt that may be the reason because there were two of us eating. I had one that had already been defrosted properly, my son had the one that was defrosted in a microwave. Of course it could have been anything but the doctor said the timing was consistent with the burger meal, but of course there could have been something in the burger (or in the air) that would have caused it however we'd cooked it.
I'm just a bit more careful now, although I rarely wait 12 or 24 hours for defrosting. I find that if I put the meat in a bag in a bowl of cold water it defrosts in a couple of hours) and then I make sure it's well cooked.