I Have Had A Rash For 4 Days Which Seems

I have had a rash for 4 days which seems to be getting worse instead of better. It itches like crazy and is mainly located on my trunk, front and back. I have tried Benadryl topical and oral, Aveeno bath and Calmine lotion, nothing is helping. I was prescribed DOXYCYCL HYC Cap 100 mg for a respiratory infection and I was also taking Mucinex DM for 5 days prior to the rash. I discontinued taking all medications three days ago. If this is an allergic reaction to the medication how long will the rash last and why would it be getting worse instead of better?