The amount of reward for your post can decrease for 3 main reasons.

1. As Dawne says someone can downvote you, this is the least likely in your case.
2. The internal value of STEEM decreases, in this case you get less SBDS in your payout but more SP.
   2a. The reverse of this is also true. If the price of STEEM rises, your SBDs will increase but the amount of invested STEEM you receive decreases.
3. People that had not yet voted; cast their votes for someone else decreasing your portion of the entire pool by adding more portions to said pool. <-- this has to do with how active other accounts are on the STEEM block-chain.
   If only 20 accounts have been active for the last week that have but all of a sudden we have an increase in user activity to 35 accounts; we still have the same reward pool to share but have added another 15 people to spread it out across. All shares will be less especially if the 15 accounts that have started to vote have a large SP holding.