Do you have a full and updated copy of the current bylaws of the league? When the board of the league meets, does someone take notes of the proceedings and publish official "minutes" of the meeting so that there is a record of what happened? When someone joins the league or the board, do they sign any type of agreement to abide by certain rules? In order for someone to help you, they will need to review the bylaws, and the minutes of any meeting at which your status was discussed, and any other governing rules or paperwork, and they will need to know both your side of the story and the other side as well.
Chances are that even if the grounds they state (violation of confidentiality) are incorrect as per the by-laws, there are probably other bases for them to dismiss a board member. You need to read the by-laws carefully to see what other reasons they could give for your dismissal, as well as any provisions regarding voting and appeals. You should also keep records of all communications on this topic to help your position (i.e., they explicitly state in a letter or email that the only reason you were dismissed was for violating the confidentiality of board meetings). You should also look to the controlling state law for any similar cases and statutes on point.
In general and going forward, however, you really should keep whatever goes on in the board room in the board room. Board members must feel confident and secure that their opinions and decisions will be kept confidential, thus enabling them to vote and act without reservation and in the best interests of the company and its members.