I secretly drank someone's drink before it was served to them
That can't be true. Why would you do that?
They kept calling me uptight.
If a drink is not yours, you should not drink it. It was meant for someone else and not for you.
Time to confront the guy that he's been making fun of me.
Just because they were poking fun at you, doesn't mean you are allowed to mess with their drinks. It is tampering, and you could get into a lot of trouble or get them sick. How about just skipping the drink, and going straight into talking to try to reconcile?