my blood pressure rarely rises over 100/63, sometimes it gets as low as 83/49 like today, i just feel extremely tired, and lightheaded- almost every day. I m 17 in a month- and my sister who is pregnant gets same readings as me- im not on any medication- yet sometimes when i lye down, i can feel my heart beat faster for a moment, its like a big thump- and then everything is back to normal again. should i be concerned?