Someone took credit for your work. They can copy the result. They can’t copy the skill that made it.

A stolen outcome is a single object, and the thing that produced it stays with you. Whoever took it now has to keep producing at that level without knowing how, which is a problem that arrives on its own schedule. That does not make it fair, and the anger is reasonable.

Keep records from now on, drafts, timestamps, messages, not as paranoia but as ordinary practice. Raise it once, calmly, with whoever can actually do something. Then get back to producing, because a second piece of work is the clearest answer available.