|Research says urushiol can penetrate rubber, so it can easily go through ordinary fabric. That is why it is recommended to wear Vinyl gloves, not rubber or latex, if you know you will be exposed to PI. Your sitting on it probably made the plants exude more oil than say, brushing by the plants as you walked.
Reaction times vary. Outbreaks can appear up to 10 days after intial exposure, and in all cases can continue to have new areas pop up 2 weeks after that. Most cases take 3-4 wks. to resolve. Severe cases and those who take prednisone usually lengthen healing time. Good luck and sit on a soft pillow :)