These are often due to some inflammation in the tendons or bursa (fluid sacs) within the shoulder. Sudden jars or falls can cause a tendon to tear with sudden weakness which may require referral to an Orthopaedic Surgeon.
Other problems are helped with medication, injections and physiotherapy treatments.
It is best to rest with the arm supported by your side and use anti-inflammatory medication if you have no stomach problems or asthma or on warfarin.
We suggest you see your GP if no improvement after 2 weeks. A useful website is: www.shoulderdoc.co.uk for self-diagnosis and advice.