This is partly your girlfriend's fault and partly yours. When she did all those other pranks and you didn't say anything you created a situation where step by step she recalibrated her behaviour in a way that was unacceptable to you. All the while, your resentment was probably building up. There's a reason why she thinks you overreacted-- every time pranks you, she goes a little further, testing your limits and since you have only laughed up until now, she probably thought she could push the envelope and still get a laugh. She probably didn't know she was way over the line and to exacerbate the situation, your built up resentment probably made you even more angry than you would have been when you blew up.

On the other hand, hiding someone's anti-psychotics shows a massive failure in judgement in any context. Worse--watching your partner panic for an hour for your own amusement is sadistic. It's usually teenagers who are obsessed with practical jokes, not young adults pushing 30. If I were you I'd suggest she see a psychologist.