Temporary rashes mark on upper body

Patient: I have donated blood yesterday in morning ( around 9:30 AM ). In evening around 6 PM rashes started appearing on my body mainly on upper body. They are appearing and disappearing whenever I am taking Avil ( tablet ).Can anyone suggest, what should I do ?EDIT:Please see

Symptoms: Rashes on upper body like mosquitoes bite. Appearing and disappearing.

Doctor: