Two weeks ago the same thing happened to my grandfather. He ended up with a high fever that burned his skin causing blisters. His blood pressure dropped and it was really scary. He has decreased kidney function (only 1 kidney) and never should have been prescribed Niacin to begin with. He ended up being told it was an almost fatal allergic reaction. He was on two antihistamines and steroids for 10 days. It took over a week fir his redness and blisters to heal.
If you've never flushed this bad you may try taking a benedryl. I'm sorry I can't suggest more, hope you find relief soon