Have you had your iron and ferritin (iron stores) levels checked lately? Many menstruating females have a problem with iron deficiency according to an internist I had in the past. Symptoms of iron deficiency can including rapid heart rate, shortness of breath with exertion, decreased concentration, poor memory, headache, and fatigue. A person can notice symptoms (at least I did) even if the iron deficiency has not yet caused any abnormal number in your complete blood count. So, I recommend you ask your doctor for these tests, including iron saturation percentage level.
Also, I wouldn't entirely rule out your being stressed, even though you don't think that you are. It could be a vicious cycle going on. Your chest felt tight, so you worry about it, which in turn, can cause tense muscles. You really don't want to take steroids too long, if you can help it, as this can potentially cause serious side effects. I had a thought about blood clot in the lung, since you mentioned long flight and surgery, but your chest x-ray and testing are currently proving your lungs are fine, correct? Did they also check you for costochondritis? That can make it hurt to breathe.
Try to think optimistically about your chest tightness- it just may improve!
Hello and welcome to the community.
The one thing that stands out as having not been evaluated is GERD. There is a type of GERD that only affects the respiratory system and can cause the symptoms you are experiencing. i would recommend that you discus this possibility with your doctors.
Let us know how you are doing.
God bless.