Probably from regular socks...especially if they were cotton or a cotton blend...cause they will bunch up, get wet and rub. It also indicates perhaps the boots are rubbing and not yet broken in and fitting well. Though the shin area is kind of a weird place for blisters...perhaps cuff area was not snug. Perhaps if rubbing, the boots are not tightening enough or in the right places...but there might be other issues of toe slam, heel lift and etc. If rubbing continues to be an issue, use a wicking liner under your regular sb sock...to elimate the friction.
And now, if you are infected and can barely walk...pay more attention...if you would have attended to the issue when you first noticed it at the 3 hour mark...or have gotten your laundry done...son get a clue.