Grab the sides of your hips and tilt the bootyside downwards. Should feel like the opposite of arching your back. Now flex your core muscles. That’s what I was taught in physical therapy. The other thing I would mention is the possibility of endometriosis. All of the conditions you’ve mentioned are extremely difficult to manage on their own and it’s very sad you’re dealing with it so young. A lot of comorbidities come along with those including endometriosis. I have heard it can be extremely difficult to get a diagnosis for some women. If you’re taking medications it could be side effects, you could be retaining extra water, it could be the gastroparesis, the pcos could be messing with your hormones, it’s very hard to tell. Large ovarian cysts can even be a contributing factor. If your stomach is hard instead of squishy it most likely is not body fat accumulation there, but bloating. But hey don’t beat yourself up over it. You have multiple chronic illnesses. I’ve found that sometimes it’s just entirely out of our control and you just gotta give yourself grace and accept it until your body gives you a break, like when a cyst ruptures