You need to check your co-op bylaws and rules. There's usually something there about who is responsible for what with leaks--depending on where the leak is, the co-op could be responsible or the owner of the unit.

The thing to know about water damage is that water runs along the path of least resistance. It also runs downhill. Frequently the first sign of trouble with bathroom plumbing is on the ceiling of the room below the bathroom. So it's not impossible that you had a leak that you never saw, and that the ceiling below your bathroom was damaged and needed to be repaired and repainted. If the leak was long-term, there may have been mold that needed to be treated.

You need to get a neutral inspector in there to determine exactly where the leak is. Then you need to check the rules of the co-op and the laws of your state--those laws vary from state to state, so you need to seek guidance from someone familiar with your state's laws.

I've always HEARD that you're not responsible to pay until you've been found to be responsible...that might even include them going to court. Not sure how that works in a co-op, however....don't you all just own shares in the entire building, unlike in a condominium where you own a particular unit?