why is my milk supply fluctuating sooo much

I'm not pumping yet. My LO is four weeks old.  But problem is past 3 days I barely had any milk. Baby was feeding every half hour all day trying to get milk. ( he lost 2 ounces everday) Then all of a sudden I was sooo engorefed yesterday and had so much milk. ( baby gained 10 oz in one day). Are soo much fluctuations normal?  For my milk and baby's weight?  What can I do to make things more uniform everyday?