Why is my right upper arm paining?

Need more info. Where does it hurt? Was there an injury? How long has it been a problem. More information is needed to help you.
Trauma? Difficult to say without more info if related to trauma, id use ice and nsaids along with rest. If symptoms persist ortho eval and xray would be indicated if no trauma, possible etiology might be a cervical radicular type pain which also should be evaluate because the treatment would be very different from upper arm traums. Definately need more info.