gaining a pound each day in a deficit. HELP

It would be helpful if you open your diary.

Unless you've been eating 3,500 calories over your maintenance each day you haven't gained a pound of fat. We all fluctuate each day, sometimes by a few pounds. I was 116 pounds 2 days ago, today I'm 112.6. Just keep at it, you'll probably find the pounds come off in the next few days.

I do agree with opening your diary though, others might be able to tell you what foods you're eating that cause fluctuations.

Concerning water retention...

http://www.bodyrecomposition.com/fat-loss/of-whooshes-and-squishy-fat.html

Opening your diary may help concerning things like sodium consumption, alcohol etc

You may also be overestimating your TDEE (Total Daily Energy Expenditure)
http://www.myfitnesspal.com/topics/show/818082-exercise-calories-again-wtf

the components of TDEE are repeated below:

BMR (Basal Metabolic Rate): The number of calories you burn at complete rest.
EAT (Exercise Associated Thermogenesis): Caloric requirements of training, or training expenditure.
NEAT (Non Exercise Activity Thermogenesis): Caloric requirements of activity that is not planned exercise. Vacuuming, driving, brushing your teeth, for example.
TEF/DIT (Thermic Effect of Feeding or Diet Induced Thermogenesis): Caloric expense of eating/digestion.
TDEE: (Total Daily Energy Expenditure) = Sum of the above. BMR+EAT+NEAT+TEF

Hope that helps somewhat OP.

Be patient, drink lots of water and make necessary adjustments where necessary if gains continue.

Read this: http://www.acaloriecounter.com/blog/why-am-i-not-losing-weight/

Water weight, it'd be very had to actually gain a pound of fat per day, you'd have to be eating way over. So watch your sodium, like others have said. Don't worry too much.

depending on your age you might also look to your cycle as you retain some extra water at certain times without any extra sodium. you may also have had more carbs or less protein which can cause water shifts too. Up the exercise and drink your fluids.

if you were undereating before and recently increased your calories, the gain could be glycogen and water, and the increase in waist size due to having more food in your intestines. Neither of these is a bad thing and the gains will stop after a while and if you are eating at a deficit, once your glygocen stores are replenished you should start losing weight again.

this isn't the only possibility and it only applies if you've recently increased the amount you eat by a lot. If you have reduced the amount you're eating then the above isn't a possibility, so see the other replies for other possibilities.