Creatine as such should not cause any allergies, since these are naturally occuring amino acids that you already have in your body. Therefore, most probably you are having an allergic reaction to some additives that might be in your particular creatine product - many companies will add flavorings and other additives to make it taste better. Hence the reason it is best to use raw unflavored creatine. Personally, I use Ancient Strength, since it has three kinds of creatine, and it has no additives whatsoever, so you are getting a very clean product with it.