My son was diagnosed with pityriasis rosea yesterday. the rash disappeared from belly today, spread to extremities, now his feet and lips are swollen.

Lasts about 6 weeks: Pityriasis rosea rashes last several weeks, often up to 6 weeks. If a person has a rash that comes and leaves in one day, it is more likely an allergic rash... which may mean a true allergy to something, or a bodily reaction with something without being a true allergy. One's primary care doctor can help sort things out.

Report: Report your son's status change to the Physician who diagnosed the rash. Rash is self-limiting, but there are treatments to minimize symptoms. Referral to a Dermatologist may be in order.