Is it under your second metatarsal head? I get same thing and was advised by foot / orthotic guy it was due to a weak or dropped metatarsal arch. He glued a small arch support underneath my insole just behind, not under, the metatarsal arch. Only cost me $15. Seemed to help but then got a bad blister under the tip of my 2nd toe so I'm still looking for solution. Interested in any other replies.
Peter