I Am Wondering Why The Last Three Times I've Had

I am wondering why the last three times I've had my blood pressure checked (before dental work) my blood pressure was lower than it should have been. Sometimes the reading was taken early in the morning, other times in late afternoon. I know I was not dehydrated as I drink water throughout the day. I've always had normal blood pressure until just recently, when it is reading low. I've also been really tired later and am wondering if that could be related to having too low of a blood pressure (it's the bottom number that is reading low by the way). I sometimes feel my heart palpitating in my chest, so now I'm worried I may have a heart issue going on. My only child was born with three severe heart defects and it's making me wonder if maybe she inherited something from me?