If it were a knee problem it wouldn't be likely to be shooting pain. If you had sciatica before then it kind of seems more likely that it's not completely cured than that you have two separate problems resulting in shooting pain, especially if the cause was your piriformis. Having a bent knee and bearing weight will put the hip in a different position too, so your piriformis being in a different position could be causing the pain even though it's not all the way down your leg.
I don't know anything helpful about how to solve it except treating it as a knee problem is likely to be a waste of time.