|Short answer: no, it cannot spread unless you keep coming into contact with the oil but don't realize it.
Long answer: you're probably very lightly touching something that touched the oil without realizing it. This could be anything you touched after getting the PI but before washing the affected area. Carpet, floors, furniture, clothes, shoes, garden tools, porch railing, garden hose, other plants nearby- anything could carry the oil. And the oil doesn't go away unless you wash it with industrial strength detergent or isopropyl alcohol. So consider washing everything that you might have touched before realizing you had PI, *and* everything that might have touched a bent or crushed or falled PI leaf outside. I kept getting it and eventually thought my allergic reaction was jumping all over my body, or was aggravated by the sun, or by pollen. It turned out my trusty garden hose, which once lay on top of a small, inconspicuous PI plant long since removed, was the culprit. I'm seriously contemplating throwing the entire hose away.