My resting heart rate laying down is 70 as soon as i go to down it goes to 90 and stays there. whats the cause?

Supine: The heart works harder to circulate blood when you're lying flat rather than sitting up. It is more noticeable on first lying down but then slows in healthy people. A persistent heart rate of 90 would be considered rapid at rest and causes such as overactive thyroid or cardiac pathology would be looked for by a physician or cardiologist.

If you mean: "when I stand up" then the answer is simple. When you stand up the heart muscle needs to either increase it's stoke volume with each heartbeat or increase the amount of times it beats each minute to make up the increased volume required by muscles to maintain an upright posture. That is the very basic answer but there are other hemodynamic reasons for the increased heart rate.