Unfortunately, it can be several things. As joints wear they can develop bone spurs (osteophytes), which can also be caused by trauma.
Several of the arthritides cause swelling in the finger joints. There are also some patterns; osteoarthritis tends to affect the distal joint (DIPJ), psoriatic arthritis the middle joint (PIPJ), and rheumatoid tends to affect the knuckles in the hands (MCPJ). Gout can form nodules within the knuckles. And, of course, there is always the very rare case of a tumor.
Without an exam and x-rays, it is impossible to tell what is going on. If it continues to bother you, you should see a hand surgeon. Good luck.