I have had the problem you are talking about. I've been having IVIG treatments for 3 years now. Last year, I began having the problem with burning skin after the treatment and even more so, burning on my tongue. It turned out my IVIG brand had been changed to the powder that has to be mixed before treatment. Once they went back to the original brand (not a powder), the problem went completely away.

I also started getting a lot worse at the beginning of the year - or maybe more accurately, I finally noticed I was getting worse. In hindsight, it was a slow downhill process that I was trying my best to ignore. Anyway, I kept thinking it was because I had too many IVIG treatments over the past 3 years, so I started cutting back (in the form of missing appts and rescheduling). Then, I crashed pretty bad. At this point, my neuro increased my treatments to 2 each week over 3 weeks and then 1 per week ever since then (with occasional 2 treatments per week on bad weeks). It has kept me from getting worse and definitely helped get me over the worst part of not being able to get up at all , breathe, or have any voice, although not strong enough to go back to work.

Hope this helps shed some light on your situation right now with MG and I hope you get better soon!