I get heart "flutters" (for want of a better description) - exactly as you describe.

My father gets them and so did his mother.

All three of us have/had low blood pressure... I have been told, as they were, that these flutterings are linked to having low BP.

Have you had your BP checked? I advise you to get a medical opinion as there are too many variables to decide from the opinion of a few of us here.

I doubt it will be over-training... I tend to have an elevated heart rate after hard training and not enough rest in between... but you should def get medical advice on this if it is bothering you.