I have a hard lump under the skin on the back of my calf, has been there for a few months and doesn't cause any pain. any idea what it might be?

Calf mass: Is it growing? It is flesh colored? Mobile? Family history? May be epidermoid cyst , lipomas-though usually soft-,dermofibroma, trichilemmoma , hematoma - if trauma, neurfibromatosis, plexiform schwannoma, myositis ossificans/myonecrosis- if trauma,rare leiomyoma or other tumor.Follow up with dermatologist first , then orthopedist if needed due to deep lesion,may need sonogram

Soft tissue mass: Most likely a lipoma which is a benign fatty tumor. However, it could be something else. I suggest you should have it evaluated by a health provider.