What's happening to my milk supply?

My baby will be 3 weeks old tomorrow and is exclusively breastfed. My milk came in 6 days after the birth and I was having no problems producing milk, my baby was eating good and producing enough wet and dirty diapers. My breasts were so full I had to pump in between feedings while my baby was sleeping to relieve my sore breasts. Over the past 30 hours or my baby has been wanting to constantly eat and has been very gassy. I tried pumping to make sure she was getting enough and I could barely get 2 oz when before I had no problem filling a 4 oz bottle and having plenty more left over. A little while after pumping I tried feeding her and I could tell she wasn't getting anything. I just tried pumping again and hardly got 1 oz, most of which was hand expressed because the pump just wasn't doing it. I don't understand what's happening as I feed on demand and I had no problems with my milk supply just 2 days ago. Is there anything I can do to get it back to where it was? Is this normal? What can I do? I'm so lost :(