What Causes Rash On Body Inspite Of Being On Zapain For Back Pain?

I have a rash over most parts of my body, this came up today. I have been taking Zapain tablets for the last 3 days, these were prescribed by my go for back pain. I have only taken 1 or 2 per day but have now stopped and today have taken some piriteze. I do not intend taking the Zapain anymore but should I see a gp or should the rash go?