Yes - do a search - there have been several threads about steroids raising glucose levels, and some people are on them long term My experience has been a couple of times with single cortico-steroid injections, and my glucose levels went up almost immediately and remained higher for several days.
Your HbA1c unfortunately will be accurate, because your glucose levels have risen, though this shouldn't have too much of an impact if the tablets are only just for a week. and your levels should come back down again once the course is finished. But it's no bad reflection on you - all sorts of things beyond our immediate control can and do affect glucose levels - it's part of diabetic life and we just have to accept that this happens. It's more important that you get the Bells Palsy sorted than worrying about a minor blip in your HbA1c.
I get slight variations in my HbA1c results all the time, and my last check was (due to stress and illness) highest since just after diagnosis seven years ago, but I know that low carbing keeps my glucose pretty well under control long term.