Account charge off

[Hecopisk]

In March of 2019 I paid off my student loan debt, in full. This morning (late may 2019) I received an email that my student loan company did a charge off on my account. It dropped my credit score 52 points! From everything I've read, a charge off can happen when you don't pay your debt and they "give up" on your account. BUT, it is already paid it off, Completely! Can they still place a charge off, should I dispute it? It took my so long to raise my credit score back up and now this one move has Completely set me back.