I'd definitely get it checked out. It's most likely either some type of cyst, which is a fluid filled sac, or it may be a ganglion, which is a type of cyst comprised of nerve cells, creating a painful little bundle. If needed, it can be removed or "busted up" with a cortizone shot. Or they might ask if you can live with it. But get it checked!
I've had these before myself. If its what I had...its like a cysts. Many oils are released behind the ears via the sweat glands...one may be plugged. I had to apply hot, wet compresses on it and, instead of squeezing it, I pulled outward around the surrounding area. It stretched it out and finally it began draining. Its kinda gross...thick, icky smelling pus like substance came out. After the initial drain, I them squeezed and MORE gunk came out. Its likely you will have to deal with these for the rest of your life...so be sure to always keep the area clean and dry. Use alcohol pads behind your ears after every shower. I once had one as large as a dime...that one I had to have lanced at the doctors office.
If you have insurance, try to get into your doctor next week or sooner if they have an opening. They might put you on an antibiotic or they might tell you its ready to be lanced, which can be done right there in the office, usually.
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Lump behind the ear could be an abscess, folliculitis (hair follicle infection), an infected cyst, an inflamed lymph node or a neurofibroma (localized harmless swelling in of nerve and connective tissue) or a lipoma. However swollen lymphnodes behind your ears could be due to chest infection, tonsillitis or pharyngitis, dandruff, or infection of ear.
Watch the lump. Do not touch everyday but once in 10-15 days. If the lump increases in size, becomes painful, changes color, bleeds, changes shape or consistency or more similar lumps appear then immediately consult a doctor.
Please consult your PCP for primary examination followed by proper referral.