It is possibly a condition referred to by some as Morton's Neuroma. There is a branch of the Posterior Peroneal nerve that wraps under the medial maleolus (the inside "ankle bone") and under the arch of the foot. It branches several more times to innervate the small muscles under the foot and the feeling under the foot and in the toes. A kind of large branch runs right between the bones of the foot (tarsals) that line up with the 2nd and 3rd toes. It can very easily get pinched. Symptoms are often numbness in those toes, or tingling. If the pain is right at the joint (ball of the foot), there is likely swelling/inflammation in there you dont see. If there is none of the numbness or tingling, it quite possibly could be as simple as a bruise (you stepped on a rock?).
Ice 20 min and Aspirin as recommended on the bottle. And get in the pool for some running, or just lay off.