Age 25 with possible rheumatoid arthritis. how common is this and how will i be affected long term?

Not uncommon: Remember that Rheumatoid Arthritis runs in families. Fortunately a blood test can clarify the diagnosis in over 80% of sufferers before there is any permanent joint damage. As an inflammatory condition it can affect all your tissues, not just joints. In your case, with a serious Cancer history, you need your Oncologist as well as blood tests to begin your search for answers. Best wishes.