The allergy what you are having is called urticaria or in general term, hives.
It can be caused by various triggers, like food additives or preservatives, chemicals, insects, pollen, animal fur, dust etc sometimes the cause is not specified which needs to be evaluated .. Knowing what is the trigger is important to avoid recurrences.
Next Steps
Contact a dermatologist online or near you for the relevant tests. And further evaluation.
Health Tips
Avoid scratching and also avoid any thing that triggers it.
Take. Tab. Levocetrizine 5mg at night as a temporary basis till you consult.