I exercise a lot, usually twice a day 5-6 days a week, and my milk supply is fine. However, I do not diet or count calories. I try to make healthy choices, but when I'm hungry I eat until I feel satisfied and I eat whatever my body is craving. Try to increase your calories and food intake, drink tons of water, and eat lots of foods that will increase your milk supply (oatmeal, flax seed meal, brewer's yeast, fenugreek, lactation cookies, mother's milk tea) and see if your supply comes back up. If not then you might need to dial back your workouts.