You are right that an injury
of the fingers is one of the more common reasons for swelling in the fingers to occur. However, it would be pretty unusual to have an injury of the fingers that caused significant swelling and not remember that injury. Therefore, I think you have to look for other potential causes.
One cause would be edema. This is a condition in which fluid leaks out of the blood vessels and collects in the tissues. It can cause swelling of any part of the body; most commonly in the feet, but sometimes also in the hands and elsewhere. Edema can be quite serious, as it can be associated with heart or kidney problems. Therefore, you will need to go see your primary care doctor
about the issue, as they will be able to rule out these serious problems.
Secondly, the swelling could be due to arthritis of the hands. There are several types of arthritis that can occur in the hands, including osteoarthritis (wear and tear arthritis), rheumatoid arthritis, and psoriasis
. Once again, these are conditions that need to be investigated by your primary care doctor, so I suggest that you set up an office visit as soon as possible.