No - I don't THINK you can gain 8 lbs in a day! You just fell off your wagon temporarily. Climb back up there, and hold this day's splurge in your memory so you remember the irritation and fear you're feeling now the next time you're tempted!
Off days are just that. Pick yourself up and keep going. A misstep isn't a dive off into the ether. You can get back on your path. Don't let it derail you.