Hi, Rachel,
You're right to be disappointed that the change has been made without consulting you, however it is the right action to take as there is a strong likelihood that the data you've added it the map is subject to copyright and therefore doesn't meet OSM terms of use for contributors.
You're right that data added which does meet those terms is then subsequently available for OSM users (i.e anyone who uses OSM to analyse the FWMC catchment in this case). If you want to continue to use OSM to hold the catchment boundary you'll need to get in touch with the copyright holder and request permission from them to release the licence so that it's compatible with the OSM licence.
A template letter of such a request can be found here: https://osmfoundation.org/wiki/Licence/Waiver_and_Permission_Templates
In the future, best practice is to list the source of the data you add to OSM within the changeset comments using the tag 'source='
I hope that clears things up for you