This is Dr. David
I am sorry you are having pain in your foot.
this pain could be due to severe arthritis or gout or a bone fracture or infection or inflammation of the fascia in the foot.
or you could have had a bug bite or spider bite there
you really should see a doctor and get a foot xray and evaluation of the area.
an xray would check for hairline fractures.
blood test can check for uric acid levels for gout.
blood test can check for elevated WBC counts for infection.
do you have a doctor you can see who can run these tests for you?