I looked Acyclovir up in my prescription book. If you have shingles, it will indeed help but a common side effect is simple rash. You may want to keep an eye on it. I had a bad ankle rash caused from a spider bite that required prescription strength hydrocortizone cream because the regular was not strong enough.
If it starts to lump up with fluid or white in the center, or begins to turn purple, or even spreads out (sort of like the veins in a leaf), it may be a serious allergic reaction to something. Mine did and it was a reaction to spider venom.
Best of luck and keep us posted!