I want to maintain my weight and up my calorie intake. Is this possible?

I currently consume 1200 to 1250 calories a day and have been for awhile to get down to the desired weight of 110 pounds. Now that I'm at that weight, I would like to consume a little more. I calculated how many calories my body will allow me to consume with maintaining my weight on this website, but I ate that much yesterday, got on the scale today and noted that I'd gained three pounds.. :( I exercise about 3 to 4 times a week on my own, and plan to get a gym membership by next week. I'll continue to eat 1200-1250 calories if it means staying at 110 pounds, but I'd really like to eat more.
Can anyone help me..? :)