MS other Autoimmune Diseases?  Unknown Rash, and Frequent Headaches

This is Monday and I have 2 more days before I have my Spinal Tap and Myelogram done to see if I have MS or another disease that mimics MS.  I do have what I think are hives or a raised rash on both arms and legs (itchy but not too bad), and buttocks.  I took Cambia on Friday evening because I had a Migrane that my Neurolgist prescribed for me.  A symptom that I have had for the last 2 months. Never had Migranes before.
Saturday, after planting shrubs in the sun with my husband I took a shower and noticed a rash appearing.  I am not sure if it was the medication, (which I have taken a few times in the last couple of weeks without getting this rash), or because I was outside in the heat.  I have never been allergic to anything, and the sun or heat has never been a problem before.
Although I feel like I have regained most of my composure and have gotten out of the funk (confusion, staring, clumsiness, forgetfulness), I now have this rash and the nagging headache pressure like if I were way up in a plane and my ears want to pop and my temples might explode.