Hi Emily,
I just got a diagnosis of POTS and Ehler's Danlos type III in the last week. I also have difficulties with hypermobility, but even after 7 orthopedic surgeries, (Five had failed) they hadn't caught EDS. I ended up having two spinal fusions, and five knee surgeries, because they hadn't been able to find out why my orthopedic surgeries were failing. The thing about EDS is that you may feel normal, like you look absolutely fine on the outside, and you may not realize you're different. I thought everyone was able to subluxate your shoulder in and out just by relaxing muscles! It stunned all my doctors, especially the doctor who fused my spine when I was a child (Scoliosis), because not even 4 years after surgery, I was so broken down below the fusion that I needed another fusion. All of the collagen in my body is either damaged or missing, and it's thought that it caused my spine to fuse, and then weaken because there wasn't enough strength in my body to keep my vertebrae and my muscles from weakening and breaking down. I was fused T3-L4, and then the revision took place at L3-S1.
Exercise is shown to help POTS in some cases, but the other big worry, if your daughter does have EDS or hypermobility in general, is the way she would exercise. A feature of both of these disorders is intolerance to exercise. When you have both, it becomes very difficult to exercise. If our daughter is exercising and has hypermobility, I would have her evaluated for the way she walks/moves/exercises, as EDS can cause damage to joints if you are exercising the wrong way.
I also struggle with low ferritin and Low Vitamin D, as well as low B12. Have you guys noticed any correlation between her abdominal pain and food? I went through an issue where I presented weirdly for Celiac, and then tested positive for it. Have you looked into food allergies?