Hello. Thanks for Posting your query to us.
I'm sorry for your present condition that is been undiagnosed for these many months.
Your doctors have already ruled out the pathology that can arise from lungs through physical examination and chest x-ray. An echocardiogram would almost never miss any disease or pathology related to the heart. There could be to 2 possibilities, In your case. The breathlessness and tightness in the chest could be due to:
1)acidity, which can produce symptoms like tightness in chest and it is aggravated by excessive work or spicy foods or lying down immediately after taking food.
2)Anxiety. It could lead to the symptoms that you have described without any structural or functional pathology.
I strongly suspect the role of anxiety in your case, I request you to consult a physician for this purpose.
Hope this helps.