What Causes Persisting Hard Lump On Labia Minora?

I have a hard, mis shapen, canal like, non palpable, mass on lower portion of L labia minora. Been there approx 5/6mts. Forms a small abscess that drains but the hard mass stays. It never changes size, only painful when abscess is growing with fuild. Hx of past bi lateral cyst that have been drained by gyn. Never had hard mass with those.