it might not be this of course)
Get a really good magnifying glass, and examine her
foot, pads, and in between the pads very thoroughly and carefully. Do it in a very good light and make sure she stays still while you do this. Also feel very carefully with your fingertips, around each pad, and between the pads.
She may possibly have a tiny thorn embedded. Sometimes, if the thorn has broken off all you will see is a small dark mark, sometimes very tiny, that doesn't wash away.