maybe she has a splinter of roots, broken glass, or wood in her feet, check them.
or maybe she cut herself on one of the three i listed.
maybe she was bit by ants, wasps, bees, etc.
You might not have saw a bug bite your pet, but it could easily happen on the sides you can't see her.
Trying to locate which part of her body is causing her pain is her first step. If her whining continues for a long while with no signs of cut or bruises, maybe it's an internal problem while trying to poo or pee