I have been having this for years. i suppose, if it were serious, I'd have dropped down dead by now.
However, I find that there are a few days per month that I just cannot function. I feel so fatigued that, if I try to do anything (including getting up and making myself a drink), my heart starts hammering and I feel lightheaded. I have tried supplements, change in diet, getting more sleep, avoiding alcohol, which has helped a bit.
Once my period is over, I get a new burst of energy. I've had several blood tests, which have come back normal. Iron was borderline once but seems to be OK now.
The GP has suggested the Pill or Minera coil (I have a copper one) but, tbh, I still had this when I was on the Pill. I;m 46 if that is relevant and I am really fed up with it now because, this week, I've had to cancel pretty much everything and lie on the sofa.
I get ghis. It's much worse when I've had alcohol or am anaemic. GP not very interested, so I've decided I'm just really sensitive to hormonal fluctuations. Which fits as I also get hideous PMT and had PND twice as well.
Cyclical palpitations are normal as I understand it (in fact I double checked that at my last heart check up as I have a heart condition). I am the same age as you and am experiencing very low energy days. In fact yesterday, I was working from home and just struggled to do anything. Today I could have happily stayed in bed all day long but unfortunately builders in the house and annoying kids prevented this. I am assuming pesky hormones.