What Causes Sciatica Flare Up?

I had a bad flare up of sciatica and today this afternoon I got a cortisone shot in the area. It seems to feel better now. The Doctor prescribed Medrol ( methylprednisolone 4 MG Dose pack) that I need to take for 5 days in5-4-3-2-1 amounts. So I started today with one at lunch, one at dinner and 2 at bedtime as was prescribed. I have felt very dizzy and have a headache since I started these pills. I think I should stop them because I am probably over medicating and not reacting well to them. Dr also gave me pain Meds which I took one but stopped. I am very sensitive to these Meds and rarely take any. Should I stop immediately all the Medrol? I can see how I do with the injection and take Advil but Dr said don t take Advil, aleve etc. withMedrol. How many hours will it take to get the Medrol out of my system and I can take Advil again? What do you think of my situation and am I doing the right thing? Thank you.