There are a lot of coexisting conditions that occur with POTS or EDS. This is not an exhaustive list, but it's a start (please comment with other conditions I should add)

• If you have POTS, look into Ehler's Danlos Syndrome
  • Are you flexible? Are your friends shocked by how far you can move your fingers, elbows, knees, shoulders, etc.? Do you bruise easily? Do you bump into stuff a lot? See my initial post about the Beighton Score
• If you have EDS, look into Postural Orthostatic Tachycardia Syndrome
  • Do you get dizzy upon standing? Do you see black spots? Do you regularly get nauseous? Can you feel your heart racing? Are stairs your mortal enemy? See my initial post about the Tilt Table Test

All over the body

• Mast Cell Activation Syndrome (MCAS)
  • This an article by Dr. Jill that has a lot of credible references and breaks down what MCAS is.
• Fibromyalgia
• Small fiber neuropathy

Neurological (Brain)

• Neurocardiogenic Syncope
• Migraines

Spine/Neck/Back

• Scoliosis
  • Tethered Cord Syndrome
• Chiari Malformation

Gastrointestinal (Stomach/Digestion)

• Irritable Bowel Syndrome (IBS)
  • There are 3 types of IBS
    • Irritable Bowel Syndrome with Diarrhea
    • Irritable Bowel Syndrome with Constipation
    • Mixed Irritable Bowel Syndrome

Joints

• Labral Tears
  • Hip labral tear
  • Shoulder labral tear

Medical Research/Presentations

• Pradeep Chopra, MD 
  • Different Types of Pain and Treatment Options for EDS
• Heidi Collins, MD
  • Here is quite a list of comorbidities for EDS (Follow the link and Scroll down to slide 37)
• Irish EDS and HMS Website (with references)
  • Short list of comorbidities you can click on to learn more