She could be over-heated...are you feeding any kind of corn feed? If so, stop. Corn is fine in the fall and winter months, but it also causes the body to generate heat. Chickens can't sweat, so they pant. If she is over--heated, they often vomit and get very listless and often die. I would separate her just in case, if you are feeding corn--stop for hot months, and give her plenty of fluids and layer pellet. Did you feel her crop? I was also thinking she could have sour crop--hence the yellow stuff coming out of her mouth. Feel her crop and tell me how it feels--is it soft,empty, hard? A good way I can tell if it is sour crop is, I carefully lean my chicken forward, head down, massage the crop upwards--this is risky, as the chicken can aspirate, so you want to do this quickly, allow her to breathe, then do it again, etc. After they vomit, I give 1 tsp of olive oil, massage the crop, etc. Let us know and I will give you more info if this is the case. Good luck!