A holter monitor is essentially an ambulatory ECG and an unremarkable holter is great news but it's only looking at the electrical currents of your heart and doesn't rule out structural issues.
POTS and IST are actually considered dysfunctions of the autonomic nervous system with cardiological symptoms rather than a cardiac disorder. IST can be diagnosed by holter but POTS can't, the gold standard for diagnosing POTS is still by means of a tilt table test although some cardiologists will diagnose through what is often termed 'the poor man's tilt' (it involves lying down for 10/15 minutes, taking heart rate, then standing up and taking heart over a period of 10 minutes usually - have a Google, it's something easily tried at home). Either way, my personal opinion is that any neuro-cardiologist or cardiologist worth their salt won't diagnose without carrying out tests (echocardiogram, cpet etc.) to rule out a structural issue with the heart.
As for whether it's just normal for and you should just accept it, my personal opinion is no. You don't know from an ecg/holter that it's not structural and if the fatigue you're experiencing is related then it will have an impact on the quality of your life.
POTS and IST aren't life threatening but they can be progressive and incredibly debilitating. I think you've hit the nail on the head in pointing out that having a baby has worsened and probably highlighted the impact it's having on you and this applies to life in general. In my experience age, stress, common illnesses can all do the same. We can't avoid these things and if you do have an underlying issue, you won't regret having a diagnosis and specialists in place should your condition deteriorate. Also, POTS and IST are treatable for many so why suffer?
More than anything, I would want to know it isn't structural though and repeat investigations now you're not pregnant would be prudent.
Happy to chat any time. x
- Very definitely NOT a doctor