I have a tender spot on the bottom for my heel. initially i thought it might be a thorn but i found nothing and it's very painful if i step on it. it's been 4months, not getting better or worse.

Further work up: I would suspect there's something going on--either a foreign body like you suspect or something else like a bony spur, plantar fasciitis, etc. I would start with X-rays and go from there. Consult an ortho doc or a podiatrist.

Heel tenderness: I wonder about plantar fasciitis (inflammation of a ligament of the foot)and heel spurs(bony overgrowths) you might want to see your podiatrist, and get orthotics and supportive footwear advice.