D.B.
Happens all the time - could be a relapse, could be something new. Either way, it's because her immune system is weak and she's susceptible to everything. Colds usually last a week, the flu can be longer (10 days), and if the cough becomes bronchitis, it can last 3-4 weeks especially because it's worse at night and the person just isn't resting well. I used to get bronchitis 3-4 times a year for at least 3 weeks at a clip, almost always from a cold that took hold in my chest. Sometimes I'd get a sinus infection, and that could last for weeks too. I always ate well (back in the day before processed convenience foods) and drank fluids, and I was the kid that played in the dirt and all that (back when we knew better than to use all these antibacterial soaps and sanitizers). But I still would get hammered by respiratory stuff until I strengthened my immune system. None of us get sick anymore, thank God. I think you're just starting to see this for the first time, and so it's throwing you for a loop. I'm not surprised, and I doubt the pediatrician would be either.