Losing weight?

I havent had my baby yet. But i was wondering if there are any mothers who BF and went to the gym? Did working out have a decrease in supply? I have to lose the baby weight, plus some because im Active Duty Army and we have regulations for weight. I have been flagged for being over weight after the birth of my 1st baby 1 year ago. Im debating if i should just do formula when hes born. Ive also heard BF mothers gain weight.