Because, since that event I get pretty red from eating foodstuffs containing niacin.
I'm pretty familiar with allergic reactions, having allergies of my own, but the flushing alone does not really sound like one. It's a common side effect of niacin and probably some people are more sensitive than others.
Besides, if you have an allergic reaction to something severe enough to land you in the ER, then it's very likely that you could not tolerate ANY of it at all. It would have eventually sent you into anaphylactic shock. Allergies that severe usually tend to only get worse with each exposure.
You ought to get a list of ingredients to the medication and see if you can determine what it was that caused the problem. Don't forget to investigate dyes and colors. My oldest daughter is severely allergic to carmine and Red Lake 7, the dye in pink Benadryl. We have to use the dye-free for her.
This a necessary vitamin for normal biochemistry and physiology. I'd see an allergist. Pellagra