What could be a cause of uncontrollable facial grimacing?

Dystonic reaction. is another possibility. Can't see what meds you're on, but read carefully any side effects and if dystonic reaction is one of them, this could well be the culprit. Street drugs can be contaminated, and do this also. Stop taking anything you don't absolutely need and go to a clinic or ED if it keeps up. Good luck!
Nerve compression. If the facial grimacing is only on one side of the face it can be due to compression of the facial nerve by an artery. Surgical treatment is available for this but would need further workup.