Why would my bp be lower when sitting then when standing for example 99/81p86 then standing 103/70p94 & then have nausea for 2 days?htn & pcos

Insignificant: The blood pressure readings you present show an insignificant difference but your question is a good one. The answer however is not simple and is a complex mechanism involving sympathetic, and parasympathetic nervous systems and hormonal systems affecting venous return, cardiac output, arterial contraction and relaxation. Any good physiology textbook can elaborate on the interactions.

Blood pressure: You are young and your sympathetic nervous system is strong. You have pain and this combinatoin raises your pulse and pressure. Try back exercises, multiple studies show they help and this would help your BP too.