Whats causing odd flavour in reheated chicken?
I will assume you mean you are microwaving your soup when reheating.

Microwaving effectively boils the water within food, mostly at the surface, and will at a minimum leave it dryer than it was before.

As for the smell, the water removed from the chicken could be unmasking a certain smell in the food or even carrying it out and delivering the smell to your nose.