One cause of low bp is acute infection or shock.in case acute infect., low bp is constant low during the day?or can be normal sometimes and others low?

Not shock: Patients with shock have low blood pressure because they are deathly ill, they can not write letters. Blood pressure is normal if you can function without loss of consciousness, dizzyness or weakness. The cause of your fluctuations is not shock.

Hypotension: Hypotension from acute infection is usually due shock septic or reaction to the infection. In either case is severe illness that may require pressor support. Low BP could be due to dehydration or normal.