Coughing up yellow phlegm. how do i know if it's from my lungs or throat? i do have a post nasal drip but that stuff generally looks lighter in color.

Your sinuses: most probably, as you would be much sicker if that phlegm is coming from your lungs, although it is still a possibility. upper respiratory tract infection is more common, and your description of post nasal drip supports a sinus problem, you may want to see your doctor, as you may need an antibiotic, in the interim saline nasal washes would help, good luck