Have you tested a similar product with that amount of niacinamide? I used a cream for sun spots that have niacinamide at around 3-4%, and it was stinging at the start (in the instructions, they even mention that the cream can cause some temporary redness of swelling, which is common in niacinamide containing products). Now, 10% is quite a lot, and if the pH is low and it hydrolyzed to niacin, it could be even more stinging.