What could this lump on my arm be? I have this lump on my left arm and I don't know what it is.... I've had it for yrs now and it get sore every now and then... Its not really visible on the outside but its just under my skin its gotten a bit bigger since

See dermatologist. sounds like you might have either an infected cyst or lipoma. would suggest you get eval and poss tx by dermatologist, esp if lesion is increasing in size and symptomatic.
It . It could be a cyst or a lipoma. You can have it removed if it is bothersome. If it becomes larger, gets red, you have red streaks, or fever then you must be seen immediately.