Could be one or more of the tendons under your arch, could be stress fractures, could just be torn muscles. How long until you ship out? If you have about 6 weeks or more, take a two week break from running, swimming, biking or anything else that stresses your feet then see a Doc if pain persists after the break. (Swimming? you say. Yes, even swimming puts stress on your feet because you flex your toes and insteps or soles of feet against the water. If the pain is muscle or tendon related, the swimming will slow healing. If it is bone related, it won't make any difference if you swim or not. Muscle injuries heal fastest, bones next and tendons a lot lot later.)