The value of your posts are based on a 7 day average of the price of STEEM. When the 7 day average goes down, then everyone's post values go down. When the 7 date average goes up, then everyone's post values go up.
The nice thing is that you are paid based on STEEM so when the Steem price increases again then your total value in USD will increase as well.
Thank you so much! That totally makes sense. I thought something silly, like I upvoted too much or something.