It's not a bunion , it's not cancer , Its not a ganglion cyst due to the location you described, ganglion cyst are most commonly on the wrist. It could possibliy be a wart. However your description to me describes a callus! A dermatologist will not biopsy it if they do I suggest you not go back! This can be treated by a podatrist but it will take some time! I suggest changing to a shoe that has a softer padding support.
I would consult with a podiatrist. It could be a bunnion, a seed wart, a ganglion cyst... without seeing and palpating it, it's just a guess.
If the podiatrist feels it's something more he'll let you know. If there's nothing to see on the surface it's doubtful it's "skin" cancer.
Just an internet thought...