I'm a climber, so I'm talking specifically about forearms. I train regularly and have been for years, so I'm pretty familiar with how my body responds.
As I understand it, the "pumped" sensation in the forearms comes from the anaerobic energy usage; it's build of lactic acid. During my training session yesterday, I experienced more onset of a pump than usual. I would say it seemed that I was getting pumped, doing exactly my same warm ups and workout as I have been doing regularly for several weeks, at about 80% of the normal threshold.
I know that if I don't warm up good, it sets in faster. But my warm up was as complete as usual. What I want to know is what other parameters can affect this? Was I low on blood sugar? Were my muscles still fatigued from previous workouts? Did the cold I had just recovered from matter?