If your parents have it, i'd wonder about an inherited autoimmune problem like Rheumatoid arthritis. A rheumatologist is definitely a good person to ask about this.
I would say osteoarthritis, but since you are so young it might be the start of Rheumatoid Arthritis. A doctor can test you for it, either an orthopedist or rheumatologist. It might be worth going.
Thanks to both of you for your help. I'll do some research on Rheumatoid Arthritis and see what I can learn.
I guess I should have also mentioned earlier that my mother and I have both had this problem all of our lives, so it's nothing that has just developed recently for me. There's no stiffness and I don't have any of the other symptoms of RA. But, I have no idea of any other explanation, so, like I said, I'll just have to do some more research.
Thanks again! If anyone has any more ideas please let me know.