Continuous yawning (or, yawning that is much more frequent than normal) says to me that there is something wrong with her crop, or her crop area. Birds yawn in that funny, neck stretching and huge gaping way to adjust their crops. For whatever reason, she feels the need to adjust it frequently; this could be something simple or something more serious. It's very hard to say.
She could have a blockage in the crop, or just past the clavicle in her body (where sadly it could not be felt by palpating). Or, she could have a tumor or growth putting pressure in the area. My rooster had this problem when he had lymphomas growing in his Thymus (this is located right next to the crop). They were quite small but were putting enough pressure on the crop that he felt the need to 'adjust' often. It ended up being Marek's disease lympomas. I am not saying that's what is going on with your hen; I just wanted to mention it because it's so hard to diagnose problems like this.
I am very nervous that she cannot stand and that she has crop problems. Her young age is also against her.... These are both signs that can point to Marek's disease, which is typically lethal and shows up as lymphomas in birds less than one year old (not always but often). That said, it's important not to take this as a diagnosis! Just trying to troubleshoot your problem. One must look at the whole picture. Have you introduced any new chickens since you brought her home? Does anyone near you keep chickens? Do wild birds frequent your yard? Any problems with any other chickens?
It's also possible that because of malnutrition, she is deficient in a vital vitamin. Or several! Vitamin deficiencies can cause neurological problems, such as leg paralysis or general drunken flopping. It's entirely possible, if this is the case, that vitamin therapy can cure her, though if she is having digestive tract issues that caused this in the first place, those need to be addressed for her long-term survival.
I regret that without more information there isn't much I can offer as to solutions, other than you can try feeding her easy to digest, soft foods, and attempt vitamin therapy to see if it helps her.
