That sounds exactly like what I experience when I take niacin, though I didn't always. When I first started taking it several years ago, I had no flushing, and so when it finally appeared, with the stomach pain, I was concerned, but my doctor told me not to worry, since it was normal, though I don't know if I mentioned the stomach pain, just the burning, itchy skin. I eventually stopped taking it, but am now taking it again, trying to find a dosage that doesn't result in intense discomfort. With the flush, I can hardly breathe, anything touching my skin hurts, and my stomach feels like I haven't eaten for a day or two.