the white paper probably goes over some of this. As far as I understand it, your payment is like a piece of a pie. The more votes you get the more of the pie you get; however, when other people vote for other posts, then your piece gets smaller and their piece gets bigger. So your payout went down because other authors/curators are also getting their piece.

I think you would have to send your steem or sbd's to an exchange, may be wroth doing a search in steemit for a guide on how to transfer (even then you will probably have to convert to bitcoin, and then bitcoin to dollars)