What causes severe pain of right inside upper arm?

Could be anything: More information needed please. At this stage the pain could be due to infection, inflammation, muscle strains, a clot in your blood vessels, a pinched nerve in your neck..the possibilities are endless

Many causes : There are many causes to what you are describing and more information is needed. Consider a telehealth consult as it may be a good starting point in determining if this is something to be concerned about. Hope that helps!