What is it called when someone writes a sequel to a book, but it's a short story? It's a like a story that happened before a book, but it's a short story?

In a word, yes, short stories can have sequels.

Here's the problem. When you publish the sequel, how do readers find the original? Even if it's published in the same magazine, new subscribers won't have the original.

When I sold the novel “Speaker for the Dead” to TOR, it was as a sequel to the Analog short story version of Ender's Game. But I found that to set up Speaker properly, I had to turn EG into a novel first, with a greatly expanded ending. My publisher agreed on a handshake at a convention.

Ender's Game turned out OK, and Speaker also worked, but I think that without an easily findable original, the sequel would not have been viable.

So unless your magazine editor wants to run your short stories as a series that you will later collect as a book, then give some thought to turning your short story into a novel, and then create sequels to that.

Explanation:

Epilogue, Its usally at the end off a book. Its a short story concluding or tying in loose ends of the book.