I did a similar thing after backwashing the day before an end of season pool party for my kids soccer team. I guess I should have backwashed a bit longer, as that is the only time it has happened to me. You can try shocking with bleach in case there is organic matter to kill. I would also turn the pump off overnight and see if brown gunk will settle on the bottom of the pool, and if it does, vaccum it directly to waste as they are likely pretty fine particles.

I now only backwash until after events!