Can someone explain this please?

No you haven't ruined it all. Give it a few days. Eat right. Exercise. Weigh yourself first thing in the morning after you use the loo and before you have anything to eat or drink.

And above all, DON'T PANIC!

Water weight from all those drinks and unless you are super pooper i doubt you have digested that food and its still in your stomach. CALM DOWN it will even out again in the next few days, mfp gives you a deficit to lose weight anyway and if anything you'll probably have only put on a pound when all is done.

yes I agree, a lot will be bloating because you are now not used to eating that much. Pick yourself up, do some exercise tomorrow and you will be back on track, it is virtually impossible not to have a slip up and sometimes that will put pounds back on, as long as more are coming off, you are on the right track.

If I'm not mistaken you said you weighed yoourself TONIGHT! Never do that because you will always weigh more at night than you did that morning. Drink some water tonight, pee it out through the night then weigh in the morning! The 4lbs should be down by then.

I agree.i never weigh myself at night.i do always right after i go potty and always naked

Don't freak out.

1) Your weight fluctuates daily, hence why it's always advisable to weigh in only once a week.

2) You've got time to work it all off.

3) There's absolutely nothing wrong with giving yourself a break every now & then.

It's actually highly recommended. It won't feel like you're depriving yourself if you allow for treats, which you did today. Maybe just a tad bit more than is best, but hey, it happens to the best of us. All you can do is accept it, track it and move on. If you feel that guilty about it, burn a few more calories than you normally would if it makes you feel better.

Just remember one day didn't get you to this point and one day won't keep you from getting to your goal

I have lost 9lbs over the last 3 weeks.. and today was my daughters birthday party. I had decided to give myself a break, have some of the cake, and a few drinks etc.. Anyway, a bit of cake turned into 2 slices, a few drinks was 8 drinks (although was pimms with diet lemonade).. and i had cheese on my chilli.. but i weighed myself tonight, and i have put on 4lbs?!?! how?? have i ruined it all?

To have gained 4 pounds in ONE day youd have had to have eaten something like 3500 calories for EVERY SINGLE POUND over what your body could use. Multiply 3500... 4 times....I seriously doubt you could have eaten THAT much food in one day. Keep in mind.......thats OVER what you burned.

Relax...you're ok and didnt ruin anything

Always, always weigh yourself at the same time of day, like others have said, first thing in the morning is best. Naked. After you pee. That way your stomach is empty, your bladder is empty and you're not adding any additional weight from clothing. Even where you place the scale can make a difference...make sure you put it on the same spot on the floor. Don't beat yourself up over one day of indulgence. You're only human. And birthday parties are a great excuse to give yourself a day off!!