Red spots with Vitamin C product

How long have you been using vitamin C and how often do you use it? It could be that your skin hasn't adjusted to the product yet so you could decrease usage frequency. What type of sunscreen (physical/mineral or chemical) do you use? Chemical sunscreen can be irritating to sensitive skin, so if your skin is reacting to the vitamin C, the sensitivity of your skin to chemical sunscreen can be increased.

my dermatologist told me vitamin C can be irritating to the skin so I avoid it.

If the spots are acne, that can happen with vitamin c. However, if they aren't acne you could be having a reaction to the vitamin c or something else in the serum.

no red spots but I put vitamin c over a pimple and it burnt. I won’t be doing that again.

Nope, it works great for me and this is the one I use. Hope this helps!