Restarting breast milk production

So I have only been able to get out 2-4 oz per day. He eats that amount in a feeding. Also his tummy was really messed up from the combo of formula and breastmilk. The other day I had a terrible allergic reaction to something we still dont know what it was. I havent pumped or given him the beast since (fear of him having a reaction). My boobs are flat and nothing will come out. How do I restart the supply? It was only 4 days but is that enough time for you milk production to dry up?