Actually the spreading is caused by ivy oils still under your fingernails or by something you had on that day or something you touched that did not get washed.

The fluid under the blisters is NOT poison ivy oils, it's your own body fluids, same as you would get with any type of blister.

Other than that, baking soda paste, calamine lotion, taking Benadryl (an anti-histamine) every 6-8 hours is about all one can do. And yes, you shouldn't scratch it or rub it if you can help it because you can cause a skin infection.

Good luck.