It sounds like your lymphnodes may be swollen and anything can cause that. Allergies (this always happens to me) water in the ear, infection anywhere in the body, big or small.
Someone asked the same question for their daughter on another form (strangly using the exact same words as you did). You can go here. http://www.medhelp.org/posts/Undiagnosed-Symptoms/hard-lump-behind-ear/show/200309
How long have they been there? If they have been there for years, they could also be what is called fat deposits. My son has a few of these - and one is below his ear (near the sideburn). The doctor told us they could be removed if they were located in a spot that could cause discomfort or may interfere with daily life (like shaving...)
Even though the have the name 'fast deposit' they are on the firm side and not squishy.
Just another possibility.