Binge

I've had days like this. It would be pretty common to see a gain of 2 pounds on the scale the day after. That would break down into 1 pound of temporary weight, water, etc and ~1 pound of fat.

So to answer your questions in order.

1. Yes you may have gained something less than a pound of fat.
2. It matters in the sense that every day matters. Is it catastrophic? Not by a long shot.
3. It's up to you. Personally, I would probably cut back a bit, but nothing too major. I'd add another 500 calorie deficit to today, more if you can do so. On the other hand, if you prefer to just adhere to your normal routine, that's perfectly fine too.

If you get back on track, then the worst thing that'll happen is a slight delay to meet your goal, I'm talking about days, not weeks. If you don't get back on track, well that's truly the worst thing that'll happen because then your goals won't be met at all.

Bottom line - fuhgettabowdit.

If you think of banking you calories on a weekly basis then you have time to make up some of those calories. Maybe eat at an extra 100 calorie deficit and add more exercise the rest of the week to help make up for yesterday. And don't beat yourself up. We've all been there and eaten over our calories.

Safest bet is just to go back to your regular plan. When I have a high day, I'm usually a little less hungry the next day. A little added deficit won't hurt if you're not hungry for everything in your normal plan. Don't severely restrict, though. That sets you up for a binge-restrict cycle. That's a headache. Eat normal to avoid a repeat binge.