What Causes Itchy Welts While Taking Percocet For Back Injury?

Taking Percocet for pain related to back injury for about 8 years 2-4 7.5 or 10/325 mg a day. Recently had a fall injuring neck and re-injuring my back. Have been breaking out randomly with streaks of welts or raised areas, hot and itchy at times but not pimples or puss these are just itchy raised areas. This has been going on for a couple of months off and on. Had injections in back last week and continued the pain medication as prescribed but I think I have narrowed these welt outbursts to Tylenol. Stopped taking these last night in favor of Advil to see if this goes away (careful to avoid any side effects so may taper if necessary). As long as my pain has subsided and I can stay away from the necessity to take these any ideas on how long to prove the theory and what would cause this sudden reaction to Tylenol of that is what this is? Benadryl seems to help.