probably just a lymph node. my sister had a pea size lump behind her ear too. it's harmless and should disappear in it's own accord. but if in doubt see the doctor!
is it hard or soft? either way, go get it checked out at the doctors. does it hurt? if it hurts it may just be a infected pimple or some type of cyst. just go have it checked out
Please dont be too worried. Alump behind the ear near the mastoid is usually an infection of the inner ear which has spread to the mastoid. Please consult your doctor right away. With a strong antibiotic regimend you can usually cure the problem. If left unattended it can cause loss of hearing and death!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!