Hello
This is normal, do not worry.
The pool of Steem that is given to comments and posts is limited, so if other posts receive alot of upvotes while your post doesn't, the value can go down with the next upvote, as upvoting refreshes it.
If you do not receive any upvotes for example, the value will still go down if you do anything that refreshes the valuation, or at the time at which you receive the reward.
Not sure if this is understandable the way I wrote it, feel free to let me know if not.