The tendon under my foot keeps cramping up. feels like im walking barefoot on pebble stones. my calf also very tight.

Plantar fasciitis: It could be plantar fasciitis. It could also be a tight achilles tendon. Usually we treat plantar fasciitis with nsaids, ice, good arch support, and stretching. We also advise not to walk around barefoot. Best of luck!

Pain: Cramps tell me it may be a neuroma. Any tingling or numbness to toes? Could be problem with plantar fascia but can't tell without more information.