Sad to say if there were tiny Monarch caterpillars on your milkweed, the soap may have killed them. For future reference (and those yellow aphids surely will come back, btw) try just spraying them off with plain water from the hose next time. The aphids are killed but if there are tiny caterpillars, they will climb back up onto the plants.
Another thing to note, I always interplant the milkweed fairly close to something else that will cover it up once the caterpillars do find it. All my milkweed plants are just sticks right now, completely denuded by the monarchs. It's what you want to happen, but it's not pretty. Gaillardia make a nice companion planting and the butterflies like it too.
