Hey u kno what? I have somethin exactly like that but in my right hand. There is a tightness in the palm and I can't straighten my middle finger (there's nothing wrong with the bone, it just won't stretch out properly and I've never broken it or anything). There is also a sort of fluidy sac under the skin that makes a bump right in the middle of my palm. It doesn't hurt when I practise except if I try to do a big stretch between my thumb and third finger.
My dad is a doctor and he thinks it may be a ganglion, which is a sac of fluid due to prolonged tension in the hand.