Why Do I Have Low Blood Pressre (as Low As

Why do I have low blood pressre (as low as 86/55 this after noon) after backing off on my BP med to 80 mg Diovan? My normal resting pulse is low 60s or high 50s. This afternoon it does not want to drop below 78.
I am feeling a bit dizzy and a tad foggy-headed, something that has been occurring for months with increasing frequency.
I have had orthostatic hypotension sice I was in my teens (now 68).
I have very slight chest pressure (not any pain, bu similar to symptoms I have had for years. On the occasions I went to an emergency room my heart always checked out fine.
Note: I was diagnosed with a very low level murmer years ago. And I do have an enlaged heart. This may be at least partlally do to my longf distance running during my 30s (30+ miles a week, many races, 3 marathons).