Re: Why does this happen?
You sound a lot like me about a year ago. I would be minding my own business, and suddenly my chest would feel tight, and I couldn't get a deep breath no matter what. My jerk primary doctor insisted it was anxiety, but I knew it wasn't, and he started treating me badly and refusing to make appointments after I refused the anti-anxiety drugs. But that's another story. Anyway, my allergist did a bunch of tests to make sure I wasn't developing asthma, and when all those were negative, ordered a CT of my sinuses. It turned out I had bad sinuses that required surgery, and a condition called sino-bronchial syndrome. Basically, I had a "stealth" sinus infection that wasn't causing other symptoms, but was draining into my chest. The reason the symptoms come and go, is because sometimes I was so congested it couldn't drain, and at other times it was able to drain easily. It was worse near the end of the day, and my allergist said that was probably because my zyrtec was wearing off at the same time.
If you have an allergist or ENT tell him your symptoms. If you don't, ask for a referral.