Was there a world chat message that said that you got 7 stars on that hero?

If not, it was likely a display bug or sync error.

If there was, use the support feature (near the chat box where it says support, yeah click that), and file a ticket with a screenshot (if possible) and keep pestering them about until it is fixed.

If you think it was a display bug/sync error, report it to support anyways and see what happens.