What could cause a recent dominant arm weakness?

Nerve Involvement. Most common reasons for weakness without other symptoms are that a nerve controlling muscles in the arm has been compressed (can occur in the neck/spine, brachial plexus, shoulder, elbow). Other reasons like stroke or loss of circulation usually have other signs such nausea, loss of pulse, feeling of cold skin or numbness. Would advise testing and visit to neurologist or pcp first to diagnose it.