It seems to be due to allergy to any of the drug you are taking. Thus stop all drugs.
If you are not taking any drug at present, it can be due to food taken or reaction to any other disease.
like TDLC, TEC & Stool test
will be required.
You may require anti-histaminic and corticosteroids
orally for few days.
Please consult your doctor, who prescribed you treatment for cellulitis
With good wishes,