Hello, my name is Dr.Dorina Gurabardhi. I understand your concern.
Several conditions may lead to lumps behind the ear. In order of likelihood, they are:
-Infection
-Mastoiditis
-Abscess
Sebaceous cysts
Acne vulgaris
Lipoma
Some advices that I can give to you are:
-Do not squeeze, scratch, drain, open (lance), or puncture the lump.
-Keep the area clean by washing the lump and surrounding skin well with an antibacterial soap.
-Apply warm, wet washcloths to the lump for 20 to 30 minutes, 3 to 4 times a day.
If it doesn't go away see a local doctor.
Hope my answer was helpful, if you have any other questions I'll be glad to help, thanks!