Thank you for asking your query.
I have gone through the description of your health issues a couple of times. And my opinion is also the same there seems nothing wrong with your heart. But there is one strong possibility related to your heart I would consider - Prolonged fatigue and malaise is very well known complication of pericarditis. The diagnosis of pericarditis is sometimes not straight forward. So a borderline diagnosis could really be it. This fatigue following pericarditis has the manifestation you have reported (except for the nausea) and is well documented in medical literature.
Now, could it be something else? Yes it could be. But these symptoms are rather non-specific. In these cases the standard approach is to consider all health conditions that can cause any immediate harm and rule out those by appropriate investigations. You Doctor has did the right thing and has ruled out the potential harmful conditions. So it really could be a post pericarditis fatigue or something like just a post infectious fatigue. You can wait for a week or two and see how it is. Also discuss with your Doctor the possibility of a pericarditis related fatigue.
Regarding specialist consultation, I recommend a Neurologist opinion. The neck stiffness and nausea is not fitting into to cardiac clinical picture. So a Neurologist may be able to give a clue. By the way I assume that common blood work like Haemoglobin level and Blood sugar has been done and found normal. If blood CRP (a measure of any on-going inflammation in the body) is not done, it's recommended.
Hope this helps.