From the Virginia Extension website...
SIZE: 1 inch (25 mm)
COLOR: Purplish-brown body with a purplish saddle shape on its green back.
DESCRIPTION: The saddleback caterpillar is about an inch long, and has poisonous spines on four large projections (tubercles) and many smaller ones that stick out from the sides of its body. The "saddle" consists of an oval purplish-brown spot in the middle of a green patch on the back. Saddleback caterpillars feed on the leaves of basswood, chestnut, cherry, plum, oak, and other trees and shrubs.
HABITAT: The saddleback caterpillar is a general feeder and is generally found on shade trees and ornamental shrubs in late summer.
LIFE CYCLE: Adults tend to take flight in mid-summer.
TYPE OF DAMAGE: The poisonous hairs or spins are hollow and connected to underlying poison glands. Contact with them causes a burning sensation and inflammation that can be as painful as a bee sting. The irritation can last for a day or two and may be accompanied by nausea during the first few hours. Usually the site of contact reddens and swells much like a bee sting.
CONTROL: A person "stung" by a poisonous caterpillar should immediately wash the affected area to remove any insect hairs and poison that remain. An ice pack will help reduce swelling, and creams and lotions containing steroids will lessen the discomfort and promote healing. Persons known to be sensitive to insect stings should consult a physician. Stinging caterpillars rarely occur in sufficient numbers to be considered plant pests, but people who work with ornamental plants should learn to recognize them and avoid touching them.
From the U of Kentucky website...
First Aid for Stings
No really effective home first aid treatments for caterpillar stings are available. Adhesive tape or transparent tape may be used to pull out some of the broken spines in the sting area. Washing the area thoroughly with soap and water may help remove some of the irritating venom. Prompt application of an ice pack or baking soda may help to reduce pain and prevent swelling. Antihistaminic drugs, often administered for bee and wasp stings, are reportedly ineffective. See a physician if severe reactions occur. Very young, aged or persons in poor health are more likely to suffer severe reaction symptoms.
That is all I have got, other than take some Benydryl or such, hope this helps!!