Sudden appearance of welts on the body

I have never had any sort of allergy in my entire life. Yesterday, I took a bite of a packet cake that felt old even though it was not expired. Within one hour, my upper thighs and upper arms were full with red welts of different sizes. I took priton before going to bed and by morning the welts had disappeared. Within one hour of waking up they welts started appearing again....on different locations on the body and they were mildly itchy. The started with small mosquito-bite like things to bigger ones and then fading after some hours again.
What can I do to get rid of these welts completely.

[National Jewish Health]

Sounds like what you ingested may have  precipitated acute urticaria (hives).  You should definitely check the listed ingredients of this food and then ask your doctor if any of the ingredients are likely to causea toxic or food allergy reaction.  If such are identified, you will want to avoid those in the future.

For now, you should skip the Priton and take a non-sedating antihistamine (prescription or over the counter) for several days.  If this fails to clear the eruptions completely, or if you were to develop additional symptoms, especially tightness of the throat, you should see your doctor without delay.

You should also report this reaction to the Food and Drug Administration (FDA)   http://www.fda.gov/Safety/MedWatch/HowToReport/default.htm

Good luck,