Your lower back?? Interesting. Anyway, Vitamin B5 (aka Pantothenic Acid) will help w/ acne -- it comes in most B Complex vitamins, but to have it make a difference you need to get it by itself & take it in doses like 3-4 x the 1 tab / day. You should see a difference in a day or two if what you are dealign w/ is actually acne. I don't know what else it would be other than acne, but just on the off chance it might be some sort of rash or allergic reaction -- the B5 does work nicely for acne tho.
Also in general, keeping your skin washed regularly, tanning booth helps to dry out acne, etc.