Right, well, stop with the formula and book an appointment with your GP as soon as possible. You need some expert advice, and possibly referral to an allergy specialist or a paediatrician that specialises in allergies.
You'd probably benefit from seeing a dietician too if your baby has a milk allergy because cows milk protein can be found in many processed food products that you wouldnt expect, and can be known as different things (such as 'whey powder' etc).
Your GP could also prescribe an appropriate cows-milk-free breastmilk substitute if you feel you need it.
That said, the canny thing about breastfeeding is that you can up your supply if you need to. It is possible to do a bit of expressing here and there. Maybe last thing at night or first thing in the morning just before or after first or last feeds, and start stashing supplies of breastmilk in the freezer. Then, for the feeds that you dont give during the day whilst at work, you can express in order to keep up your freezer/fridge stock of milk. Under the circumstances, you'd be better of sticking with breastmilk as long as possible.
When do you have to go back to work?