I have the same symptoms and I was diagnosed with dysautonomia (autonomic dysfunction) particularly Postural Orthostatic Tachycardia (POTS). There are several types and drug/alcohol intolerance is a symptom as well as heat/cold intolerance and many others.
They do not know what is causing it but according to dinet (on the web), parasites can be a factor and I am taking that route. I start Humaworm soon. Other causes are viral, adrenal problems, bacterial infections, etc... I've already taken viral medication and it didn't work so I'm convinced it's parasitic. Hope this helps.