There is a kind of lung test, I forget the name of it, with a methacoline challenge that can test for asthma.
I had that test, years ago, and that helped me to stop wondering if I just wasn't breathing right (I knew I wasn't panicking), especially, since for the most part, I don't wheeze. I have been told in the ER that my chest is "tight".
Otherwise, try to relax and think of ways to avoid/get out of the situation, that can help your breathing, even if it is asthma.
As far as eating shellfish in the doctors office, if they suggest you do that, they will probably have plenty of medicines and treatment options to get you feeling well again. Look on the bright side, it will help you know one way or the other, and deal with it accordingly. Otherwise you are left with what-ifs and no treatment options.