I would guess infection. If you can get your hands on her, high does of penicillin and my vet recommends Lutalyse. Other wise if you have dart we use an excede/sterile water combo. 2cc excede/1cc water.
I am usually in the pen with her when this happens. If she is laying down when I get there, she gets up for her treat and white milky discharge comes out. I have even seen her clean herself afterwards and sometimes go back to the spot and clean that up also. but she is not acting any different, she eats comes to me for her treats....