A tender bump suddenly appeared om thumb. could this be rheumatoid arthritis?

Highly Unlikely: If your joint is actually swollen it may be athritis but rheumatoid would be unlikely. If the bump is nit a joint, then it definitely not ra.

Maybe: There are several causes for the development of a tender bump. It could be a ganglion cyst, mucous cyst, rheumatoid nodule or osteoarthritic nodule. I good physical exam and some diagnostic tests will help identify the etiology.