I've a difference of 30-40bpm on standing after sitting, whenever I stand or I climb stairs, I can feel my fast heart beat in my head in a very painful way, but as the heart rate becomes normal, pulses in my head end, at times on standing, blackness comes in eyes for few seconds and I've lots of leg pain, could this be pots? (ECG and echo normal). I've fibromyalgia and celiac disease too.