I second MrsK's commentary. Congrats on dropping down but I would not think 110 is a healthy weight for a growing and developing 16 year old at 5'4. Dancing, swimming, yoga... These are all exercises that will strengthen your core and could help tighten your tummy :-) good luck
you are a perfect weight for your height, your BMI is 21.8. if it is just your tummy you are worried about focus on core workouts. belly dancing is easy and very effective.
talk to your doctor about your period, normally when your period stops it is because your body does not have enough fat stored to maintain the grow and development of a baby. but only your doctor will know for sure.