I've been playing bass for about a year now. And lately I've just began to notice that my thumb muscle Sears pain if I play for very long periods of time. This happens when I'm doing Iron maiden on Pick. I have also noticed some light brown hard spots on my thumb. I don't know if they are callusces, or if my muscles are getting inflamed. Does anybody know? I know for fact it's not my posture because even if I hold the pick very lightly it still does it a little bit.
I'd recommend that you stop playing for a few weeks and work on some basic hand, finger and wrist stretches during that time. Here is a link to illustrate those stretches:


It's very important to avoid "working through the pain" when it comes to guitar or bass playing. A lot of people make analogies between athletes and musicians, but it's just not the same thing. You should never feel any pain other than sore fingertips occasionally. If it's a deeper pain, there's likely a problem.

If the stretches and rest don't work after a few weeks, you may want to go and see a doctor. When it comes to pain, as a musician, it's better to get rid of it at an early stage. Playing through it only makes it worse.
If it only happens when you've been playing for a long period of time, just stop when it starts to happen.