Pain in my left foot, next to ball and arch on the inside of foot

Recently ran on saturday 23K and felt great. Set to run on sunday, 16K with run club. First K in, had a sharp pain in my left foot instep. Felt like I had tied my left shoe lace too tight. Loosend it. Did not help. Quiet painful, in the arch area. Slowly pain decreased and managed to finish 16K( not as strong). Decided not to run monday. Decided to weight train, put on my running shoes and went into the gym and right away felt like I had tied my shoes too tight again on my left foot. Made laces very loose. pain disappeared. Did not do any lower body weights. Tried to go to yoga class and did not finish the class. Could not balance standing on my left foot, but could do so very easily on my right foot. Dedided to stop right there and then. No pain at night. No pain right now. I am running a half on Oct 11. Next 4 weeks are crucial to my training? Need help? What do I have?

Thanks for your help, Joe