What Causes Chest Tightness And Breathlessness?

Few months ago I had a random episode of breathlessness and tightness in the top right of my chest which seemed to move into my right arm, I felt weak and so breathless ( as if I had ran a marathon !) just prior to this episode I had just been relaxing and walked downstairs when these symptoms occurred so I was not even exerting myself in anyway. About 1 month to 6 weeks later (not entirely sure) I developed a chest infection for which i had 2 lots of antibiotics and was then told my chest was clear and chest X-ray was ok, however, 8 weeks on I have not recovered fully, today I had the same tightness and pain in right of chest and left of chest and pain under left breast, I am tired on a daily basis and have random bouts of breathlessness which seems worse in the morning as I rise and perform daily tasks. Please can you advise me on why I am unwell. Thank you.