What Causes Painful Bumpy Rashes On The Groin Area?

My sister, 65 yrs., has what appears to be either a rash or a sequence of bites directly in her groin on one side. The marks are close together, in a row along the groin crease. She has had sciatic pain on that side also, and some sensation of heat from (including) her hip and down to below her knee. Each mark is somewhat scabbed looking and about a quarter inch at the largest. Some are smaller. They are painful especially if bumped or scratched. There is no itching. She has had it for about 3 days. There is a slight pus pocket, very small, in some of the marks, and a tiny spot in the center which is barely seen. She can t tell if there are 2 as with a spider bit, but she thinks it s just 1. We are stumped and haven t seen this before, especially on just one side.