Ground nests are the worst. I stepped into one by my wood pile a few years back. Same experience. The first sting just felt like maybe a branch or something poked me, but then it got worse and I witnessed about 10 hornets swarming my leg. I ran like a mad man into the house. It's not fun stepping into nests. I actually had a colony form in one my hugel beds this past year. I'll need to figure out a solution come spring. Good thing I have a bee suit :D