Feel very tired, lightheaded when standing. Not enough energy to get through day. Erratic BP. What is wrong?

Possible dehydration. Have a checkup and get what are called orthostatic blood pressures. They will measure your blood pressure in different positions. If the numbers come out a certain way then, you may be dehydrated and simply need to drink more fluid. Perhaps, medications are too strong and causing intolerable changes in pressure. How about anemia, low functioning thyroid? In general, you need a good checkup.