Why am I spitting up phlegm after going to China?

Well if you have drank their water, that's the reason. Sometimes people get colds through airplanes because of the continuous air flow. Also you might not have adapted to their atmospheric conditions which has caused this. Another reason is you have picked up bug that your immune system isn't use to, if i was you I'd go to a international medical centre if you could find one and ask for antibiotics otherwise try and get over the counter flu tablets :)