are always a concern. If you continue to have this problem I would strongly encourage you to see your primary care doctor
As you know, the hand is a complex instrument. Although we often take it for granted, it is an extremely complex and intricate system of muscles, nerves, blood vessels, bones and ligaments. The coordination between these components allows us to do many things with our hands. All of this is to say, any small change or problem in any component can effect something else. For this reason there are specific surgeons
who just operate on the hand...its that complex.
As for what you have -- its hard to say. The most likely cause is a hematoma. This is a problem where a bruise occurs inside the muscle and blood causes the lump. This should heal itself, however as discussed above this can cause problems with neighboring nerves or ligaments. Another possible cause is a broken bone. This is often accompanied by inflammation in the area -- which can cause the lump you described. Sometime minimal fractures -- such a stress fracture can be hard to notice.
Regardless of what the cause is, an evaluation is needed to make sure it does not interfere with other important structures in the hand. Good luck!