Very painful foot after running last time, could it be a stress fracture?

Maybe: If pain or swelling continues the foot should be x-rayed. If it only hurts when you run or right after you run it may be a shoe or mechanical problem.

Possibly: The more common stress fractures occur close to the balls of the feet and slightly above it. If there is pain every time you try to run, it would be much better to obtain x-rays and confirm, rather than to worsen the stress fracture and allow it to displace (break more and fall into a bad position).

Maybe: Having pain after(or during) activity is not a good sign. There is a possibility that you have a stress fracture, it could also be many other possible conditions. The first thing to do is back of on your activities, allow the symptoms to resolve, then slowly progress back into activities. If the symptoms do not resolve or return quickly, you should be evaluated by your doctor.