I have Orthostatic Blood Pressure. It runs either high or so low I get dizzy. Passed out once. The fluctuation Can be related to the heart issue. I take a med to raise it. Then it goes high. Doesn't mater what I am doing.

Have you told your cardiologist or your GP about this? It may be your heart function. or if you have a pacemaker perhaps the pace is set too low. That was my experience anyways.
My pacemaker was reset to give me a range, enabling me to exercise.
See your doc, also be your own advocate, do your own homework, research your symptoms. The Mayo Heart site is awesome ( a .org) meaning they aren't trying to sell you anything, but there is so much good info on that site.... I highly recommend it.
Good luck. :)

I'm the same, okay until 11ish in the morning then it's bed in the afternoon. Fed up.

I know the feeling of having zero energy. If you do d the cause of a solution Please let me know.

I would talk to the nurse at your doctors office. It could be the drugs your doctor put you on.