Well, if you are using tobacco in your hookah, that might explain the coughing and the brown plegm. That said, you are using PG and VG in your mixes, you may be sensitive to inhaling one or the other. you might try mixing up your ratio, or even just VG and see if that helps. I would still go see my doctor if I were you, just to be on the safe side. Certainly, it may have been the juice, there are certain ones that really don't play well with *my* chemistry, either I find them revolting or they just don't work.
One can find cases of walking bronchitis/pneumonia, I had it for 8 months the second time, and I didn't feel *well* exactly, but I didn't really realize it was serious. I didn't feel that awful, and it took about 4 visits to get it figured out because I wasn't crazy wheezy, etc., so they gave me various allergy meds. It took a while to get everything sorted, and then my third bout was actually trying to quit, on too much pg, and then I got pneumonia once more... So there are cases of vg sensitivity, and it may be worth seeing your doctor again and going, "Hey, take this bit of my lung sputum (I know, gross) and get it analyzed to see if it's a bronchial issue, or just coughing from continued hookah which you said you used fairly frequently. Have you vaped without hookah use? Because, one of the first things I would try is to lay off that for a wee k or two, then start isolating issues with your vape/doc.
Best of luck, stay safe!
Anna