foot question

My mom has been experiencing bad foot pain over the past year or so. She works in a grocery store and is on her feet for 8 hours. She is only 43 though. She complains with heel pain and the ball of her foot hurting, she says her foot hurts all over most of the time and she said that throughout the day that her foot gets SMALLER! I thought that seemed strange because usually pain causes swelling, but she thinks her feet get smaller througout the day, she said that she has worn a size 7 1/2 shoe for years and recently that size has become too big for her feet so she got a half size smaller which fit and then throughout the day the shoe got too big.
Can you tell me any possible causes of this. She has seen a podiatrist once and he gave her a cordizone shot and said that she maybe had heel spurs or something, but the shot did not work. Is there anything else she can do for the pain. What is causing this pain and foot shrinkage?