Don't undo all the good you've done with a short fall off the wagon. It's alright. We all fall off once in a while, but we get back on again. That's what is going to make you successful in the long run.
Yup, go back to the gym tonight. Do your usual workout, don't overtax your body to punish yourself for your overreating. Take those lbs back off by monitoring your food intake and exercise. They'll come off!