Hard marble sized lump under skin right next to and in my belly button. it hurts to the touch but that's it. what could it be?

Cyst or LN: Could be a cyst or a lymph node. A lymph node would be concerning in this location, but given your age it is unlikely to be that. More likely a cyst. These sometimes get red, painful and rupture. If this gets progressively larger or you have signs or symptoms of weight loss, fever, chills, nausea, diarrhea, etc you should see someone.

Soft tissue mass: It could be a benign fatty tissue known as a lipoma, an overgrowth of skin called a granuloma, or an infected hair follicle. Apply some over the counter antibiotic ointment and leave it alone. If it gets larger or more painful, get an evaluation.