I accidentally disturbed a nest in the ground of sweat bees while weed eating. They swarmed over me and the weed eater. I have to say, their stings were very painful, 6 to 8 on my legs, hand and arms. This was not expected, and contradicts some of what I have read since. Swelling and severe pain was experienced. These were brown/black with yellow stripes, 1/2-inch in length.
My first thought is that you may have encountered something other than a common sweat bee. Typically sweat bees live in the ground but they don't "attack" or swarm like you've described. However, they look a lot like a yellow jacket and the two are commonly mistaken for one another. If you compare a SWEAT BEE and a YELLOW JACKET, you'll note they have a similar pattern. Additionally, there are different species of both distributed throughout the country and many are so similar it's easy to mistake one for the other.
Another type of bee it could have been is the common DIGGER WASP. They can and will sting and though generally not aggressive, they do group up in great numbers different times of the year. Regardless of what you have active on the property, I suggest you treat the nest using the products and treatment methods listed in our Digger Wasp article. If you now think they're yellow jackets (which commonly nest in the ground too), treat them as explained in our YELLOWJACKET CONTROL ARTICLE.
Sweat Bee Picture: http://en.wikipedia.org/wiki/Sweat_bee
Yellow Jacket Picture: http://en.wikipedia.org/wiki/Yellow_jacket
Digger Wasp Article: http://www.bugspray.com/articles99/diggerwasps.html
Yellowjacket Control Article: http://www.bugspray.com/article/yellowjackets.html