The adverse symptoms from Niacin are typical and should not be a cause for concern. We are not aware of any relationship between Niacin and modification of a wasp sting.
Putting the Niacin aside, the reaction to the sting is worrisome, especially since the natural history of recurrent wasp or hornet, also called hymenoptera, stings is that the reactions can become increasingly severe, even to the extent of anaphylaxis. The doctor did right in giving you the EpiPenĀ® (epinephrine).
You might want to check with your doctor to find a board certified allergist with a special interest in bee and wasp sting reactions, who might be able to reduce the intensity of reactions with desensitization.