First off, I would suggest you never BP without a safety rack/pins or spotters, just to be safe.
Why do you think you can do the Smith!?, and not a reg st bar BP. It sounds like it maybe something in your bar path execution, since the Smith is very dictated and straight upwards!?
Have you had other troubles with a shoulder or shoulders?
Is it only one side that acts up?
Did you ever injure it?
Can you vid doing a slow empty bar?
Otherwise, I would suggest, if you cannot do it, to steer clear.
No pun intended but:
What did Dr Vinnyboombots say to Rodney Dangerfield...
Doctor, doctor, it hurts when I do dis!?!?
Den don't do dat...!