Breastfeeding help!

My baby is three weeks old and is possibly going through a growth spurt. All morning he has wanted to breast-feed and is consistently sucking and will fuss and cry when I pull him away. I took him off and let him fuss for 20 minutes and tried to pump, I didn’t get any milk. I am concerned I’m not making milk and that’s why he’s demanding to eat for so long because he’s not getting anything. Anyone else experienced this and have any tips on what to do?