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Increase in weight can be due to excess intake of food , lack of exercise , change in hormonal
status , steroid
intake , oral contraceptive pills
As long as your BMI is normal , your weight gain is not a problem.Please check your BMI @ http://www.healthcaremagic.com/Body-Mass-Index-Calculator .
Please the change in weight also for a better advice.
You should start practicing exercise like walking , brisk walking , jogging , going to gym to make this weight gain a healthy weight gain.
Please feel free to ask any further query.