Please do not put mustard oil in the ear. It is not helpful and actually has the opposite effect.
Pain in the ear may be due to multiple reasons. A cold in the past? Trauma to the ear? Wax? Fungus?
You must visit a doctor so a complete examination of the ear nose and throat would help find out the cause of the ear pain and he treated accordingly.
Next Steps
You can give a pain killer SOS, but cause of the pain must be ruled out.