Welcome in H.C.M.
I have gone through your query regarding feeling tightness i back of throat during breathing in old air.
Since you had recovered from stomach flu
so this is some thing different, Possibilities are-
1- Throat infection
2- Cardiac problem.
Keeping these things in mind if you do not have any irritation and pain in throat than sore throat
may be ruled out and it will be wise step to get yourself checked by cardiologist.