niacin reaction or something else?

O.k. I have been getting sort of dependent on the 6 hour energy shots which I admit is not a good thing, but it has been a weird couple of days for me. On Monday, I had really bad diarrhea- so bad I had to go home and I spent the rest of the day dealing with that. I figured it was some sort of intestinal bug and I didn't want to make it worse, so I left the caffeine alone for the most part. So this morning I was feeling better, but I got very little sleep, so I gulped down a 6 hour shot and hopped in the shower. Five minutes into it, I had horrible cramps and shortly after my face started burning and turned bright red. The feeling spread to my chest, back, and abdomen, and even parts of my arms and legs. It burned like you would not believe for about 20 minutes, then went away - well, almost. I figured it was a niacin reaction, but there is only 33 mg in a shot plus it is almost twelve hours later and my face is slightly red and sensitive to the touch - like an irritated sunburn. I have vowed not to drink these again but could that tiny bit of niacin really have caused such a reaction?