Sister recently found a small dime sized lump on the right side, behind her ear, a little down. it is uncomfortable if she presses down. worry?

Likely a cyst: Probably either an inflamed sebaceous cyst or a lymph node. Does she have any ear pain or a recent cold or any irritation or cuts/scratches on her scalp? Soemtimes if it persists with pain, you could try an antibiotic if there is a local infection going on. Try some motrin (if a healthy stomach/kidneys) and a warm compress and see how it goes n the next few days, if persist, see an MD.