There are 2 major points that will impact the number that you see in $ rewards.

1. the rewards pool: the way you make money on steemit is that some steem is newly created every day, and the given to the people that receive upvotes. That amount is fixed, so if people with a lot of steem start upvoting more than they used to, then the absolute rewards decrease, since you have to give the same amount of money to more users. Currently nothing like that is happening, which leads us to reason 2.

2. The amount of $$ you receive is a function of steem price. The more steem price rises, the higher your rewards, and vice versa. You can see the steem price here https://coinmarketcap.com/currencies/steem/. To prevent market manipulation, rewards aren't calculated on the real time price of steem, but on an average of the last few days; you can check for that here https://steemnow.com/ (upper right quadrant). As you can see, steem price is 1.10, but the average of the last 3 days is currently 1.05. Overall, steem price has been down last week, so that's why your rewards, and everyone else's rewards are going down. That's jsut how steemit works :D