Red dots on roof of mouth and sore throat and rash on arm?

May be infectious: This may be a viral exanthem or strep throat associated with a skin rash. There are infections that may affect the mouth and throat and skin. You should see your doctor right away for cultures and possible treatment with antibiotics if necessary.

Allergic reaction?: You need to check for a possible food or medication allergy, log longevity of symptoms, onset, recurrence, take photos. Start with md or dermatologist.

Virus: Could be a secondary hermetic outbreak. Most people had the chicken pox as a child and have the herpes virus lying dormant in their system. A herpes outbreak in the mouth can look like little spots with a red ring around them. Give it a week to ten days and it may be gone.