Blood pressure at 138 when sat down, stand up and it drops to 92/62 dizziness and fainting also, what causes this? Bp naturally low so 138 high for me

Change of BP. Your symptoms sound as if you have orthostatic hypotension (op), which means your BP falls when you stand up. If this is happening often, you need to go to an er and be evaluated. Causes of o could be volume deficit, loss of fluids, or an irregular heart rhythm, among others. Good to an er and get the answer to your question from the physician who examines you. Good luck.