It shouldn't hurt during, but I find that it gets a little "extra sensitive" afterwards.
The idea is that one encourages "more" growth if one pumps it while it's growing. Mine tends to appear "fatter" after each pump but then that simmers down in 20 minutes or so.
However, I've noticed that it's "fully pumped" size has grown significantly during the period that's passed since the first time I pumped.
My pattern is.
10 minutes pumping, 10 minutes rest, 10 minutes pumping, every morning and evening. (When I get up, and when I get to bed).
I use a little alarm timer, and then read or meditate while I'm pumping.