by arch, do u mean on the top of the foot or the bottom of the foot where the arch is? either way though, it sounds like u strained some tendons and maybe even a couple muscles. the same thing happened to me a while ago and just got better (by a while, i mean middle of XC this year, i got a very large acorn stuck on a spike and it didnt come off for two miles. by just got better, i mean like maybe a month ago) its gonna hurt u for a while if its anything like what i had but it isnt really anything to worry about as long as u ice it and take it a bit easier on it. like maybe do some XT on a bike or sumthing for a few days just to be safe.
but worst case scenario it sounds like it could also be a stress fracture. does it continue to hurt u while u run or does it go away while running and then come back maybe an hour or two when ur done? if it hurts while running its probably a stress fracture, if not then its probably just the tendon thing i described. if ur really worried, go get an x-ray done. just take it easy for a few days then see wat happens