Teen with finger/finger joint swelling but no trauma happened. what could it be, and what treatment?

It depends.: There many different causes of atraumatic finger swelling. The amount of swelling and duration would help narrow the possibilities. If swelling is significant and doesn't resolve in a reasonable period of time then it becomes very important to seek further medical evaluation. An examination followed by appropriate diagnostic studies (xray, mri, blood work) would be important.