Is it behind the ear or on the ear if it's on the top part of the ear just on the lip its a cartilage over than that go see your GP and it could be nothing to worry about
Probably a post-auricular cyst.
Without any description it is hard to tell.
Usually it not a cause for concern, as most lumps behind the ear are
insignificant. A few may require medical attention.
Please provide more details, like size, hard or soft, painful or not, how long
you had it, any other symptoms or dermatological (skin) issues, etc.