I have had ME/CFS for about a year now, and have developed problems with orthostatic intolerance. Initially my OI symptoms manifested as light-headedness and feeling very crappy after I'd been standing for 15-20 minutes, such as when doing grocery shopping. It steadily got worse, until I could only handle 5 minutes or less. Typically my pulse pressure would get narrow, and my blood pressure seemed to drop when I measured it while standing up for a while. My pulse rate didn't change much from standing up. This seemed to indicate Neurally Mediated Hypotension, which is pretty common with ME/CFS. But about two weeks ago, I had a very odd episode which I think might have signalled a transition into POTS. I had been standing up for 10 minutes (too long), and was startled by seeing my fiance in the window as he came home. I didn't have the typical adrenaline response, however. 15 minutes later I suddenly felt really awful. I felt like I couldn't control my legs well enough to keep standing, and same with my arms for holding a bowl of food. My throat felt tight/rigid, but not swollen. I was able to breath fine, and I was also sweating (something I never do usually). I didn't feel any better after sitting down, and had to lay down. After laying down I started panting and I wasn't able to sit up again for the rest of the evening without feeling awful. Getting to the bathroom was quite scary due to instability. I continued feeling the same for the next 4 days. On the fourth day, I went to the doctor and she couldn't get my pulse or blood pressure on my left arm after 2 attempts, but a nurse taking it on my right arm got a reading of 135/75, although she said my pulse was very weak/soft. The doctor was not concerned that I frequently have a pulse pressure in the low 20's, and said a pulse pressure of 20 is completely normal. She sent me for blood tests, including electrolytes, the typical blood stuff, liver function, etc, that all came back normal. The next day I was able to sit up again (with my feet up), for up to an hour at a time, though I'd then be feeling quite bad and have to lay down again without being able to think clearly. The rigid feeling in my throat continued to occur. But taking my blood pressure has given different results in the past week. Sometimes my pulse now rises 20-30+ beats per minute, and I have a narrow pulse pressure less frequently. Here are some readings I took tonight (systolic/diastolic, pulse (position): 6:00 - 123/100, 111 (standing) 6:05 - 131/ 87, 92 (sitting) 6:10 - 123/ 86, 82 (sitting) 6:15 - 131/ 86, 80 (sitting) 6:15 - 115/ 85, 78 (lying) 6:20 - 118/ 80, 77 (lying) 6:25 - 118/ 94, 95 (standing) 6:30 - 114/ 97, 84 (standing) I wasn't able to get any additional readings due to getting an ERROR message, probably due to too many missed heartbeats. The rigid-throat feeling was only present during the first reading, when I also had a rapid pulse, so I'm wondering if that symptom was a POTS thing, and if it sounds like I have POTS and/or something else going on. Any thoughts?