I am experiencing dizziness that gets worse with sitting upright, that is unaffected by changing head position, exercise, hot weather, lying flat, prolonged standing, and that gets better with rest, severe back pain in the lower part, spinning sensation,

Dizziness: If you're experiencing vertigo, a spinning sensation, upon sitting upright from a supine position, an inner ear disorder is likely. Rolling over in bed causing a spinning sensation is also characteristic. Low back pain seems unrelated. Any recent strain? A lightheaded sensation on standing up is seen with low blood pressure or even anemia. Getting examined by your MD is advisable. ?see ENT also.

Dizziness : Dizziness when sitting upright is likely from a loss of blood pressure or circulation. This can be from dehydration, moderate to severe anemia and several other things. Inner ear vertigo typically comes from changing head positions which you do not have. You should see a doctor in person