http://www.nancymohrbacher.com/articles/2012/11/27/how-much-milk-should-you-expect-to-pump.html
Your emotional state. If you feel upset, stressed, or angry when you sit down to pump, this releases adrenaline into your bloodstream, which inhibits your milk flow. If you’re feeling negative and aren’t pumping as much milk as usual, take a break and pump later, when you’re feeling calmer and more relaxed.