You should definitely go see your primary care doctor
or a sports medicine doctor
about this issue as soon as possible. They will help you figure out if there is still something going on with your finger that needs treatment or evaluation.
When you 'jam' your finger this typically means that you sprain the ligaments between the various bones in the finger. This leads to swelling and pain, which can last for weeks, but should eventually go away with no permanent damage done.
The fact that you cannot fully extend your finger is concerning for a more serious ligament or tendon injury
. For example, something called a mallet finger is an injury in which the tendon at the very tip of the finger becomes detached, which prevents the end of the finger from extending out all the way.
The treatment for mallet finger involves putting the finger in a splint for a long time. Unfortunately, this splinting treatment works best when it is started right after the injury. It sounds like your injury occurred quite a while ago, in which case there might not be anything that can be done, but you should get to your doctor right away to find out.