Hi friend,
Welcome to Health Care Magic
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What you are describing is ARRHYTHMIA.
Treadmill exercise ECG (TMT) is done to exclude Ischemia (reduced blood flow) – it does not address arrhythmia that is not caused by ischemia.
YOU NEED FURTHER FOLLOW UP WITH THE CARDIOLOGIST
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Anxiety / Alcohol / Tobacco (smoking) / Caffeine (too much coffee, cola) are common causes.
Medicines may be responsible – like phenylephrine used for ‘cold’ / Salbutamol used for asthma / Illicit ‘drugs’ like amphetamine are notorious to cause abnormal beats
At times, there are abnormal nerve paths (bypass tracts) in the heart.
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Some times, they may be normal. Yet it is advisable to have it investigated.
A thorough history and physical examination is the first step -

If a routine ECG does not show, 24 to 48 hour AMBULATORY MONITORING (Holter) is done to analyse and plan for further management

ECHOcardiogram is necessary to see the heart valves (Mitral Valve Prolapse) / heart muscle (Cardiomyopathy) and heart function (Ejection fraction, wall motion abnormality).
Laboratory work-up is routine - like blood counts, sugar, urea, electrolytes (potassium in particular), thyroid (over-active) and so on. Your Internist shall suggest based on the situation.

If there are no clues and if you are still bothered by it to the extent of interfering with life style, there are more tests. These are advanced techniques of investigations of increasing complexity (and invasive) – like Electro Physiological Studies (EPS) > it resembles angio / a catheter is put inside the heart, electrical activity recorded, stimulation and suppression tests are carried out, suitable medicine tested and so on. Though the test is the gold standard, it is invasive and has a minimal risk and is not generally done unless there are compelling indications. This super-speciality expert is called ELECTRO-PHYSIOLOGIST. The treating doctor may suggest them depending on need, based on his assessment of the situation.
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These are not always symptomatic.

Specific treatment may not always be necessary.      
The first line medicine is generally beta blockers, like Metoprolol – slow release and small dose. It will benefit both anxiety and missed beats. Your doctor will give proper prescription and follow up

If they are not effective and if the problem is severe, several other classes of drugs are available – acting by different mechanisms - the specialist will decide tailored to your needs / they may have side effects and need follow up.

There are other advanced treatment modalities - like RF ablation, which could be done during EPS.
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Sorry for the long treatise!
Relax. You are young; you sound intelligent – you stand an excellent chance of returning to normalcy.
Be positive – why fear the heart beats fater? The problem iis only if it could not beat!
You are tolerating the rate well - means that you may not have any basic disease of the valve, muscle and so on.
Be in touch with your doctor - it is a question of time and you will be well.
Medicine is well advanced nowadays – management is possible for almost all situations...
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Take care
Wishing speedy recovery
God bless
Good luck

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