Oh my god, this is weird.
My Mum has the same, and I literally said to her this morning, that I would ask on here about it... thanks for reminding me!
I know you can do stretching exercises, but these didn't work for my Mum either.
In fact, she had a hospital appointment yesterday, where they injected the bottom of her foot with some kind of steroid injection.
Spoke to her about an hour ago, and she said it was still hurting. However, these injections are supposed to be good, so maybe speak to your doctor about it.
I know that investing in a good pair of cushioned shoes helped her... even though she's been limping for the past few months.