Yeah. medically, there is no connection, unless like GuitarRox says, you're doing it all day and not exercising or eating well.
If you're gaining weight rapidly, without other explanation, see your doctor for a work up. You could have a hormonal or thyroid issue.
I sure hope not! There is not going to be a link between gaining weight in your midsection and masturbation. Change up your diet an increase your weight. Are masturbating all day every day to change the form of your arm muscle? This does sound like a stress situation for you. Masturbation is pretty harmless but if you feel it is negatively impacting your life, then walk away from it. Go on a diet, get some cardio and life some weights (if your doctor okays that kind of activity).