There are a number of possibilities in this case.
It maybe something as simple as impacted ear wax.
Or, it maybe an acute infection of the middle ear.
The diagnosis can only be confirmed after examining the patient
Next Steps
You should take the opinion of an ENT specialist. The earlier the diagnosis is made, the sooner the treatment can begin and give the patient relief.
Health Tips
Please do not put oil into the ear or try to clean it with a bud, as commonly practiced.