The possibilities I can think of in your case are Infectious mononucleosis
or an allergic reaction
If it is due to infectious mononucleosis, caused by Epstein Barr Virus, the infection is self limiting and there is no need to worry. It should resolve on its own in another week or so. Meanwhile keep yourself well rehydrated with plenty of warm fluids to drink. Do steam inhalation 3 times a day. Warm saline gargles 3 times a day. Have healthy and nutritious food.
You could try applying calamine
lotion twice a day on the face, arm, back and legs. You could also take an antihistamine like cetirizine
to bring down the rash
and itching. Avoid warm /hot baths which will worsen the itch.
Hope this helps.
Dr. Divya Kuttikrishnan