Blackout when I stand up?

Hey guys! :) I am a a 15 year old girl, and have a quick question! Okay, so let me give you a little bit of background. My sister is 17, and starting passing out when she was around 14. She also blacks out, and all that, so one day, after her passing out three times while I was holding her up, she got transported to Primary Childrens. REALLY scary day for me (nobody was home when I got home, then my mom called and told me. We cried, and I had to take care of my little brother all day the next day). Anyways, she got a cardiologist, and NO ONE know what is wrong with her. They think she might have hypertension, or Pots disease. :( Anyways, she said this all started when she got her period (Sorry TMI, but apparently it's because of all of the hormones), and I just started mine a couple months ago. Anyways, I have never had this happen before (except for just a couple months ago, up to this point), but everytime I stand up, or sit up, or even sometimes when I am just standing, I get VERY dizzy, and everything goes black. Usually I will have to grasp onto something to keep me from falling. I am VERY scared that this will turn into what she has. Is this normal, or should I be worried? Thanks for your help!