I have severe on side of left foot and underneath for past two weeks two x-rays show nothing any ideas?

Plantar fasciitis?: It is likely that you have plantar fasciitis--inflammation of the ligament of your arch; toward the back-end of it. Typically xray will not show, but a good history and physical exam by your doc will provide the diagnosis. Use good arch support, good stretching of the arch, and motrin/eleve for couple weeks. If not better, consult your doc. Good luck.

Foot pain: If pain continues. You might need mri.

Pain left foot: First , see your podiatrist for an evaluation. Taking serial negative x rays means it is something that cannot be seen on plain film x ray .