Thank you for your query.
Upper respiratory tract
infection (common cold
) is usually viral in origin and one can have rashes appearing over the face and/or body following it. Your son seems to be having it and the rash should disapperar in the next day or two.
It is very unusual to see a rash following MMR vaccination
and that too after nearly 3 weeks. Chickenpox
rash is in the form of blisters and not bumps as you have described.
I hope I have answered your query to your satisfaction.
Wishing your son all the best.