I wouldnt rule out that it could be an abscess, nor a kick/bang in the field. How often do you bang yourself, and it really hurts but apart from a bruise theres no over sign of it being there, so in my opinion, horses must get this too. I would keep an eye on her for a couple of days, see how it goes. Depending on how lame she is/temperament, possibly chuck her in the stable, if there no improvement within a couple of days then I would get the farrier to have a look in her foot.