drastic and sudden drop in milk supply

Hi ladies. I'm hoping some anecdotal stories can help me out here. My lactation consultant and I are trying to get to the bottom of why my milk supply suddenly cut in half!!! I'm not pregnant nor have I gotten my period. It's been a week so I don't think it's due to ovulation as it would have bounced back by now. I have been under stress since returning to work but last week I was starting to get my groove again and managed about 6 hours of restful sleep. My morning supply is still good but by evening and late night I have practically nothing and baby is so frustrated. Last night I gave a bottle of expressed breastmilk from Friday's sessions at work. I feed him 6x per day and pump 3x. What gives?!! I eat oatmeal every day, drink the teas (ginger and mothers milk) I eat fruits and veg,  Peanut butter and whole wheat bread for snacks, Chicken and fish. I am 15 weeks pp and so sad this is happening to us! I worked really hard to prevent this. I'm so upset! I had such a great supply I stockpiled 40oz of milk in the freezer. I can start dipping in to that, but I don't even know how much to offer him after a feeding. An ounce? Two? How do I know how much to bring if we are out? I don't want any of it to go bad if I offer too much in the bottle. When I run out of breastmilk, How much formula do I use? What formula do I buy?