Some people just naturally run low, but if this isn't normal for you, you should go get checked out. One possibility is that you are dehydrated. This leads to a condition known as "hypovolemia" (when your body doesn't contain enough fluid to keep the circulation at a steady volume) and this can lead to lower blood pressure, especially when you go from a seated to a standing position. That is called "orthostatic hypotension." Next time you take your BP, try it when you're lying down and see if it goes any higher. If it drops when you stand up, this might very well be the case.
It could also be a million other things, or nothing, but this is the first thing that came to my mind. The only way to fix it is to start drinking plenty of electrolyte-enhanced beverages like Gatorade or Pedialyte. Get some salt in your diet (salt helps the body absorb water). And definitely bring it to your doc's attention the next time you go for a visit. Good luck :)
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Please always remember to consult your medical professional regarding your medical questions; this forum is intended to provide patient-to-patient support. Although some of us have healthcare backgrounds, we cannot diagnose or treat patients on the board.