Severe IBS leads to malabsorption issues, and I've seen people (and animals) start losing weight because of it, even when they were able to eat normal quantities of food. She's had terrible diarrhea, and anxiety at not being able to find restrooms in time while out in public, lest she shit herself (which just tells me she's been shitting herself), so that could be it.
Uncontrolled diabetes can also cause rapid weight loss (see: Josh Campbell in Polissa's thread), and it's often the first big red flag that leads to a diagnosis. Corissa's mentioned being on Metformin, so she's been at least insulin resistant/prediabetic for a while, and in one video she did actually say she was diabetic. But despite being a heavy consumer of medical care, she ignores any medical advice that involves putting even reasonable restrictions upon what she eats. She didn't change her diet when she had her gallbladder out; she hasn't changed it despite ongoing gastric issues; and maybe she really is insane enough to keep on stuffing her face with whatever she chooses despite being a full-blown diabetic.
And yeah, it could be any of a number of cancers, because obesity is a high risk factor.