I am experiencing low blood pressure (hypotension), dizziness (better by: rest) (worsened by: changing head position, lying flat, sitting upright, ...

Needs to be. Evaluated. A number of hormonal issues could be involved: cortisol; thyroid. See your doc. By the way: giving your BP would help!
Postural hypotension. Go see a good human doctor and give a thorough history and get a complete physical exam which will guide any laboratory tests to be done. Then sit down and have a long detailed discussion about what is going on and why and what your outlook is and if he/she is unsure get a referral with a copy of all of your data. Make a virtual appt. Check www.mayoclinic.com before you see yr doc under postural h.

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Underweight, low blood pressure, fast and slow resting hbr, achy legs and arms, breathless with stairs and dizzy spells. Is it my heart?

Don't know. It could be your thyroid, an autoimmune disease, just your heart, a lung/heart circulation problem .... See a physician and get to work at getting to the bottom of it. Read more...
M.D. visit. A first step in the management of these symptoms is to have your cardio-pulmonary condition assessed. Pulmonary arterial hypertension should be considered in the work up. Read more...

What to do if I have been dizzy, nauseous and have low blood pressure. My ear is also clogged. Is this serious?

Related. Your clogged ear is very likely the cause of your dizziness and nausea - the symptoms go together like peas and carrots. You likely have a cerumen impaction and serous otitis media. It can be more serious. Some viruses are damaging to the accoustic nerve and result in permanent hearing loss. Vertigo and nausea slowly subsides. You need an ENT exam as soon as you're able to arrange one. Read more...

I do have history of vertigo and low blood pressure. Is it normal that my head gets dizzy for a few second once I get up from a sitting position? Mostly when I m kneeling on the floor. Also, when I lie down flat on bed...my head can feel a bit dizzy. But

No. But with the history you describe not unexpected. Would also be concerned about benign positional vertigo. This is a benign condition that can be evaluated and treated with some simple maneuvers called Epley maneuvers. You can Google a video search or YouTube search this and try them on your own to see if they help. However, continue to do what you're doing, change position slowly to prevent dizzi. Read more...
Yes orthostatic. This is probably orthostatic hypotension - the body needs to adjust when you move positions. I suspect this si why you feel dizzy. Your physician can clarify and be certain that your blood pressure is Ok Also your labs like blood counts are all OK and that there are not any circulatory or inner ear issues that could also be playing a role. Read more...
Probably normal. However, you want to visit your doc and ask about a tilt test, meaning you are a little upside down and then your position will go up. No big deal but better be safe. Good luck. Read more...
Fluid status. It's common for one to feel lightheaded upon standing when one is dehydrated, and there are other causes as well. Check your blood pressure sitting and standing; if it falls below 100 systolic (the upper number) with standing, try drinking a tall glass of water, then repeat the BP check sitting and standing. If this continues to recur, see your physician for an exam. Read more...