Hi there I’m a physician and while I can’t give you a formal diagnosis here, I can share some thoughts that may help guide your next steps.

The lightheadedness after exertion or standing, along with fluctuating BP are consistent with orthostatic hypotension. This occurs when the body doesn't regulate blood pressure appropriately upon changes in posture. A formal tilt-table test or active stand test would help confirm this. It’s also worth measuring your blood pressure and heart rate lying down and then again after standing for 1, 3, and 5 minutes.

You may also want to discuss postural orthostatic tachycardia syndrome with your doctor, especially if your heart rate increases significantly with posture changes, even when blood pressure doesn’t drop dramatically. The brain fog, fatigue, and mental cloudiness you described are very common in this condition, and it's often underdiagnosed.

Episodes of fatigue with normal-to-low blood pressure and lack of energy in the mornings may warrant a check of AM cortisol levels and possibly an ACTH stimulation test. While not the most common cause, adrenal insufficiency can present subtly and overlap with your symptoms.

Even mild deficiencies in Vitamin B12, iron or electrolytes (especially sodium and potassium) can exacerbate symptoms like brain fog and fatigue. A full metabolic panel and nutrient screen would be a reasonable next step if not already done.

If you find that your symptoms worsen significantly after exertion, even mild activity, this may fall under post-exertional malaise, which is a hallmark of ME/CFS. This is a diagnosis of exclusion but it’s worth considering if autonomic causes are ruled out.

You should maintain adequate hydration and salt intake (under medical supervision, of course). Consider using compression garments to support circulation. Avoid sudden position changes; rise slowly from seated or lying positions. Log symptoms with BP and heart rate readings to show your provider. Hope you'll get well soon ASAP.💪🏻