I can assure you that you can't gain 7lb over night! a lb of fat is 3500 calories, you'd have to have consumed 24,500 calories to gain that and a pint is about 200-300 i think.
it is likely that some of it was fluid retention if you drank loads of water to re-hydrate?
I'm the same as you - i weigh myself every morning without fail and can gain stupid amounts over a few days. it does me no good though and funnily enough, it does generally drop back off after a few days. try not to let what the scales say worry you and just put your efforts into making your day healthy.
Regarding trying to lose a few lb - why not try cutting down on your starchy carbs? i'm not sure what you generally eat but i find that if i cut down (not omit! Atkins did me no favours when i tried it years ago!) on starchy carbs, i retained less water. eat more salads and protein, and keep drinking water - if you give your body water, it desn't hold onto it so much
hope it helps and all the best with your goal x