It is caused by:
taking up a new form of exercise, suddenly increasing the intensity of your exercise or a change in surface (eg road to track)
standing on your feet for several hours each day
medical conditions such as rheumatoid arthritis or systemic lupus erythematosus SLE
wearing high heeled shoes, or switching abruptly to flat shoes
wearing shoes or trainers that are worn out with weak arch supports and thin soles
having flat feet or an unusually high arch
having legs of uneven lengths or an abnormal walk or foot position
having tight Achilles tendons
being overweight
There are measures that can be taken initially to improve pain in the feet:
avoid any exercise that is making your pain worse
a heel pad is sometimes used to cushion the painful heel if you spend a great deal of time on your feet on hard surfaces
over-the-counter insoles which fit inside your shoes may be advised to address specific imbalances you may have
you will be shown stretching exercises to do three to five times a day which can help
you may be advised to apply a protected ice pack to your heel or to use an ice block to massage the plantar fascia before going to bed each night
If you are still troubled by ongoing pain, then I can recommend a local podiatrist who undertakes full biomechanical assessment to identify the cause of your pain and who can provide you with custom made insoles to help correct foot posture.