I am feeding my 3rd child and out of nowhere my milk is in short supply... ?

I have not changed anything in my diet. My daughter is now 8 wks old. I was producing 5-6 ozs when I pumped and now I can barely get 2-3 ozs! I was not giving any supplements at all and trying to pump in between feedings to produce more. I have tried drinking more water as well. What is going on??? I was so stressed at my daughter frustration and hunger that I finally gave her 1 oz of formula yesterday to 'top her off' and I feel horrible now. This is my 3rd child... with my first I stopped since it was so painful (I got thrush and once I started supplementing it was a gradual decline and stopped at 3 wks.) with my second child she wound up having a protein allergy and stopped after 2 wks. After 7-8 wks, although its hard, it was working out for this child... and now this. I have heard from friends that they had to stop because of their milk supply not increasing and was hoping to work through this, but what gives.... If this is mother nature's way and the most natural... why doesn't it come naturally??? Any suggestions on increasing the milk supply.... Please help??