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.
Probably a post-auricular cyst.
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