Why do my lungs ache after I carry my backpack?

(24 year old male, approx. 5'10" and 60Kg.)

I've been travelling with a backpack for six months. In mid-April I developed chest pain while travelling through the snowy eastern part of Turkey. Initially I put it down to the cold; venturing outdoors certainly made the feeling worse. My lungs felt heavy and constricted, and lifting my backpack (weighing less than ten kilos) caused considerable pain across the inside of my chest. That evening I developed an intense stabbing pain in my upper left abdomen every time I took a breath. I've experienced this sensation before, perhaps once every six or eight months, but it usually clears up within five minutes if I sit absolutely still and take shallow breaths. However, after trying to relax for three or four hours the pain was still just as intense. I became worried and found a doctor. He listened to my pulse and chest (no cough, no discharge) and said he thought I had either pleurisy, caused by a virus, or else a myalgic muscular strain. He instructed me to keep as warm as possible and prescribed both muscle-relaxant gel and some anti-inflammatories. Neither seemed to have a noticeable effect but, after finding an extremely warm room, the pain eased off and I was able to sleep, though the feeling of breathlessness and the 'heaviness' in my chest and arms persisted for several more days.

I've been trying to carry my backpack as little as possible since then, and also to lighten the load. It's now less than 5 kilos, but even after carrying it for five or ten minutes I'll usually feel a tightness in my chest for a day or so afterwards. I'm in Vietnam at the moment and the temperature is above 30 degrees, but two days ago the breathlessness and heaviness in my chest returned.

At the moment my lungs feel 'heavy' and breathing causes more effort than it should, though the pain isn't much worse during inhalation. I feel breathless when walking, and I am most comfortable when lying flat on my back. Additionally, my left arm feels 'full up,' as though someone is squeezing all the blood downwards towards my hand. This is most noticeable when walking.
I've woken up several times in the past couple of days with a racing and heart and very strong pins and needles sensation in my hand and arm, as if they were completely empty of blood. When I was 18 I was diagnosed with an irregular heartbeat and the arrythmia has been much worse than usual in the past two days,

Over the same period I've also had bad indigestion and lots of trapped wind.

Has anyone experienced similar symptoms after carrying a backpack? I have no idea whether these symtoms are pleuritic, muscular, circulatory, caused by a hernia, etc.
Does pleuritic inflammation of the lungs sound like a good explanation, and can it be repeatedly triggered by carrying a backpack for even short intervals?

I intend to visit a doctor as soon as I arrive at the nearest city (Da Nang) but in the meantime I'd very much appreciate any feedback about what might be causing these symptoms.