Thanks. It used to go numb when I was using a bracelet which was compressing my blood vessels. Then I started avoiding tight clothing and any bracelets and it went away. Some positions may also make your hand numb, for example if your arm is raised al the time (depending on where and how you draw). Some people hold the pen really tightly, this can also cut the blood flow to some fingers and make them numb. I try to hold the pen lightly.
Mmmmm, I think I see now why it happens to me, I'll try to fix the way I use my pen, maybe I'm holding it too tight. Yes, I might get too tense unconsciously since ballpoint pen is kind a little unforgiving. Thank you for the advice!