"Outstanding shares" in a coop?

I very recently finished closing on a coop and just received my first maintenance charge. On the letter from management is an assessment for "outstanding shares" which is equal to my maintenance. So I'm wondering- first, what are these outstanding shares that I have to pay an assessment for and furthermore do I have any rights given the fact that no one- not the broker, my lawyer, the board, or the managing agent informed me about this assessment or how long it's going on for? I'm concerned that I've just closed on an apartment that I now can't afford because this information wasn't shared with me. Any and all advice is much appreciated, thank you!