Possible mastoiditis? there's a lump behind my ear, somewhat tender.

Lump behind ear: Most lumps behind the ear are either an enlarged lymph node, possibly infected (if it is tender) or a sebaceous cyst (a sac of cheesy smelling oil and fat), which can be tender as it enlarges. Mastoiditis does cause tenderness directly behind the ear, but is not usually associated with a lump except in severely advanced, untreated stages, which is unlikely.

Could be: Depending upon the size, location, color, texture, fluctuance, and mobility, it may be a number of things. Masdoiditis is among them. Often you may get a cyst behind ear, in the crease.