What Causes Fluctuating Blood Pressure Level?

Hello, Last May, I had a mild heart attack. I am 46 years old. My troponin levels were elevated, a catheterization showed no blockage. My BP was 175/115. Since then, I have lost 15lbs and take a beta blocker sporadically because I would really like to be off of it. It seems to make me depressed. Anyway, I have noticed that my BP is 150/100 sometimes when I am sitting, but drops to 130/80 or so when lying flat. Also, occasionally, my BP will be almost level at around 120/110. I don t feel bad at all though. My question is, What does it mean when the systolic and diastolic readings are so close and why does it drop when lying flat? Thanks!