just stretch your hand out when you play. you will get use to playing longer sets. Also, You will probably see more pains in your hand if you are in to playing Bass fast with your fingers. I (seriously..) just close my eyes and deal with the pain or "soreness".
Its just as if you were lifting weights and you feel as if you can't lift on the ninth rep but you concentrate and are able to do it. As time goes on, you could peobably do 20 reps of that same weight.
Just give it time.
