Why would i have.low blood pressure?

Many things: If you have low blood pressures, especially with systolics in the 80s or below, and this isn't your baseline blood pressure, you need to get evaluated by a doctor immediately. Low blood pressure is an ominous sign of things such as volume depletion, infection, or heart failure, and cannot be properly diagnosed without an exam.

Blood pressure: If we make the assumption you are otherwise normal, you may just have baseline stroke volume and peripheral vascular resistance that yields a low but normal pressure. As soon as we allow abnormal considerations low blood volume, low stroke volume, low resistance and reasons for them all come into play.