How likely is it to gain weight after a diet?

The other day I ended my diet because my family was getting worried that I was too skinny, and they try to get me to eat more. Now this happened on Thursday, so I've been worried that I put on weight, maybe like 2 or 3 pounds. I'm worried because I was finally satisfied of my weight and now I'm scared I put it back on, and I don't want to continue to put it on. I looked up how many calories I should eat to maintain my weight. Also when I do splurge I do the 100 workout (40 jumping jacks,30 sit ups, 20 squats, and 10 push-ups,which is supposed to burn 100 calories but I don't know if it's working, and I'm just really worried.