I see that you are currently offline, i will leave an initial answer here which you can read at your leisure and then we can discuss it later once you come online.
The first thing that comes to mind after looking at your symptoms is that you could have an abnormal rhythm of the heart. This is known as a cardiac arrhythmia. The palpitations are one of the features of the arrhythmia. Now arrhythmias are f various types, ranging from atrial fibrillation to serious life threatening ones such as ventricular tachycardia. It is difficult to say which one you might have without a EKG to determine it. It is very common to have atrial fibrillation, this is one of the commonest arrhythmias however it can only be diagnosed after a clinical examination and a EKG.
Arrhythmias can have serious consequences, in long standing arrhythmias you can develop clots in the heart which can lead to serious complications such as strokes. In addition arrhythmias can also cause sudden cardiac arrests. At the same time many arrhythmias do remain benign such as an occasional extra beat as in premature ventricular contractions. Now coming to your other symptoms such as chest tightening, this can be due to the arrhythmias itself or it can be due to other cardiac issues such as ischemic heart disease. The arrhythmias themselves can also be related to the ischemic heart diseae, this can occur due to clogged arteries.
In addition i would also like to look at your heart valaves because in many young patients, a faulty heart valve can cause all of these symptoms.
Lastly as i asked in the additional information i would also like to get your thyroid function test done as well whioch can cause the tiredness, abnormal heart rhythm a swell as the shortness of breath and the chest pains. Increased thyroid function is associated with all these symptoms.
My recommendation would be to keep seeing your cardiologist and get the following tests done, EKG, Echocardiogram (to look at the heart structure and function), Holter monitor (to monitor your heart rhythm for 24 hours) and then proceed from there.
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