Do not start rubbing/touching/trying to clean anywhere you think you made contact. ASAP, get in a shower as hot as you can stand it, and wash off with a degreasing dish soap, like regular blue Dawn. Scrub well. After rinsing off, you can change the shower temperature and shower as normal. Wash clothes in hot water and add some kind of degreasing cleaner. I like to use Mean Green.
Poison ivy spreads through Urushiol oils, and the hot water and dish soap will remove it. Often the reason poison ivy spreads after washing is because people don't remove all the oils and then end up spreading it when they scratch the itch because regular body soap is not as effective at removing the oil as dish soap.
I've been doing this for years now and IF I get a rash, it's usually very small and I can scratch to my hearts content and not have to worry about it spreading, just don't scratch so much it opens up and becomes infected.