Ain't no muscles in underside of the finger itself, so it's tendons. If the pain is gone after a week it's probably a mild sprain, you can climb moderate difficulty problems for awhile (2-3 weeks) then on to the harder 'uns. You're lucky to be young, in a few years that sort of injury will set you back 6 weeks.
Oh, and tendons are more important, without them no amount of muscle will do you shit. It's tendons that take a long time to develop, any meat head can get strong muscles.
Me, I'm too smart for that!