At my last check up, I forgot to tell the doctor I have rheumatoid arthritis. Is that ok?

You should tell him. It turns out most people who tell their doctor they have rheumatoid arthritis don't really have it. In other words, a lot of people think they have rheumatoid arthritis but don't. Hopefully you are in that situation. Normally the medications you take for rheumatoid arthritis would alert him that you have that. If you have redness, hot, stiffness in your joints see your doctor again about it.
No. You should call his/her office to appraise of your condition.