Many times Poison Ivy/Oak will blister like this. The socks you had on may have allowed the juices to stay on the sock and you got a big dose of overexposure. Walking on the plants would release the juices and the socks acted like a wick to absorb and hold them. By chance have you changed washing detergent? Some times new detergent will break out body parts exposed to the washed items.

I assure you that the surgery will be postponed because of this. Don't hide it from the Dr. This could be bad news if the surgery happens with the blisters you have.

Good Luck.
B