A co-op is a co-operative corporation in which you are a shareholder and as such a "partial owner" of the corporation.
When you purchased the co-op, you received or should have received, a copy of the Proprietary Lease and by-laws of the corporation. This lease and accompanying by-laws most likely codify when and under what circumstances changes to the maintenance charges and additions of assessments can be made. Most co-op boards generally meet at a pre-determined time and, after notice to all shareholders and following the guidelines in the Lease and By-Laws, impose necessary increases in these fees. Most by-laws likely provide for a time during which, after notice by the Board, that the shareholders can discuss any impending increases and vote on them. Very often, shareholders who have the time and inclination will seek to become elected members of the Board so that they are fully involved in the decision making processes that affect the shareholders.
A careful reading of your Proprietary Lease and By-Laws, by yourself or an attorney, will likely inform you of the procedures and timing to contest any fee increases that have already been made and also alert you of the procedure for future fee increases.
Should you want to discuss this further, please contact me off line at (516)621-6300 x 205 or (516)410-3594.