I have a moderate pain in my left foot- bottom of it, on the inside side, close to and somewhat on the heel. what could this be? (i do play sports)

Plantar fasciitis or: Possibly a spur. I would initially try a commerically available shoe insert that protects the heel but an xray or MRI may be beneficial.

Plantar fascitiis. : I must compliment your description - you made the diagnosis easy. Stretch both before and after sports. Achilles tendon stretches are good. Wear arch supports or orthotics. Ice the bottom of the foot. Antiinflammatories if not contraindicated. Massage that region of the foot before getting out of bed in the morning. If pain persists see a foot doctor.

Plantar fascitis: If related to injury need exam to rule out stress fracture. Most common cause of heel pain is from plantar fasciitis, a pull/sprain of the plantar fascia from where it attaches to the heel bone on the btm of the foot. Txs include rigid arch support to decrease the pull of the fascia, combined with antiinflammatory medication (oral and/or injectable) to reduce the inflammation.