What is your map's name?
If your map has the same file name as another map then sometimes other people, who are actually on a different map, can creep in. A common example of this in practice is when a roleplay updates it's maps, yet people on the older version will still be in there.
People who use Watermill as a base for maps and do not give it a new file name will often run into other people. Non-unique names such as "Forest" "Town" " Island" can also have this problem. All of my map's file names contain my username, so I can be sure it is unique and nobody will interrupt me.
This file-naming issue leads to users running into each-other and arguing over who made the map...when really they are in two different maps that each made and the server plopped them in together because the file names are identical!
The other issue is if you make your download public, then other people will come in. I don't see any public maps up by you in the forum though.
If someone is claiming they made your unique map, then take screenshots of them saying it, and proof the map is indeed yous and PM them to a Mod or Admin to deal with.