I’m 18, so it won’t have anything to do with the long-term health problems like you mentioned. Basically everything below my elbow feels at least a little off, but at the moment it’s worst in my elbow, the underside of my forearm near my wrist, and between my thumb and first finger.
Hopefully that provides a bit more context.
Please don't assume youth automatically guarantees good health.
At 18 we are all invincible. That's not a dig on you personally, but a fact of brain development that we all go through.
The armed services have much more success at recruiting 18 yo than they do someone in their early twenties. That is in part to feeling like nothing can harm us at that age.
Rheumatoid Arthritis (R.A.) is your grandpa's disease, yes?
I was diagnosed at 25. At that age I had just fathered my first child, never mind being a grandpa.
There is also a juvenile version of Rheumatoid Arthritis called J.R.A. This develops in younger people.
These are systemic diseases, meaning they can affect more than just joints, other organ involvement can happen.
These are also diseases of the immune system. That can lower your natural defenses against other diseases. Being exposed to someone with the Flu or Covid becomes a riskier encounter for someone with certain types of Arthritis.
Also not saying these are what you have, but am suggesting you see a doctor. What you experience does not seem normal, even for most older folks. If it is something chronic like R.A. or J.R.A it is usually much better treated and controlled when caught early. I wish they had the treatments available today, when I was first diagnosed.
Also know that this kind of diagnosis can take some time to get right. A family practice doctor may do an initial evaluation, but getting a proper diagnosis for whatever is going on may require one or more specialists.
Hope it's nothing and just a temporary condition. But if it is affecting you as you say, there is something going on, and it may be something simple. But if not, continuing to try to play in a manner that causes pain isn't going to fix anything.
Good Luck in getting this resolved.