Is this diagnosed by some expert already? Piriformis is just one of the reason that might cause the pain. I would not irritate nerve by pain by rolling on lacrosse ball. Especially if you dont know wether it is trapped in piriformis or not. In my opinion, best approach is to avoid all activity that clearly might cause irritation. You dont have to be afraid of exercise and movement, but everything should be pain free and done with caution. Remember to do stretching and mobility exercises especially really slowly and painless, dont rush for the results.