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You are actually underweight
with BMI only 17.9. Normal BMI should be 18.5 to 24.9. You should have proper diet habits and exercise to stay fit. Avoid fatty, fried, sweet, junk food. Avoid eating large meals. Rather eat small frequent meals. Eat diet rich in proteins
like pulses, eggs, chicken, lean meat. You need to eat plenty of fresh fruits and green leafy vegetables. Drink enough water. Do brisk walking exercise everyday. Since only your abdomen has become bigger, it could be sign of hormonal
disorder or probably ovarian cysts inside. This is just a possibility. Try to stay fit by the above mentioned methods. You can consult your Doctor to rue out these problems if you feel your tummy has grown recently.