I think you may have hit on your own answer! I've heard of that condition, but unfortunately, I haven't heard what the "cure" is. If you could give your thumbs a few weeks rest and see if that helps, you'd probably have your solution. I just don't have any idea what MIGHT happen permanently to your thumbs if you continue to "work" them so hard. You could talk to other game players and see if they've ever had this condition and what they did about it. But I still think if it continues you should at least talk to a doctor about it. Repetitive motion on joints CAN cause a number of problems...….some of them lifelong. I wish you the best and hope you find an answer soon.
How long has this been happening? Have you recently taken up playing the guitar or another instrument? Is it possible you are sleeping in an odd position where just your thumbs are affected? (I can't imagine what that would be!) I'm sorry to even mention this possibility, but I'd be remiss if I didn't...…..it could be juvenile arthritis and you need to let your parents know so this can be checked out by a specialist. Wish I had a definite answer for you and hope you find an answer soon.