What could cause foot swelling throbbing constantly and oain when any weight is put on it for awhile?

I assume there has been no trauma as the cause. If you can pinpoint the throbbing pain & swelling to a joint,especially the big toe, acute arthritis caused by gout would be most likely. Other possibilities could be a stress fracture, cellulitis ( a bacteria skin infection) or other inflammatory joint conditions. You should be examined by a physician for xray,accurate diagnosis & proper treatment.

I would recommend seeing a podiatrist for this problem. Specialize in foot and ankle