Last year (or was it the year before?) my DH was mowing the lawn and got swarmed by yellow jackets. He jumped off the tractor and ran for the door (which was closed and locked.) He was already stung at least 15 times on the legs. Well, DH usually uses the gasoline method as well, but this was so bad that we had to call an exterminator. The yellow jackets make more than one entrance and exit to their underground hive. The exterminator (2 gentlemen with nothing but a powder in some old oil-can looking things and no protective clothing) puffed this stuff around and in the holes. They came back at night and dug up the nest which was the size of an old 32" television, extending under a large rock. There was also a second nest buried close by. We have not had a problem with yellow jackets in that area since then.
PS I was locked in the house, hiding in the bedroom, watching from the window. Bock Bock Bock.
I suggest you call a professional exterminator, so they can locate the hive and remove it as well as treat all the entrances and exits.