There could possibly be some blood in the phlegm. Brownish phlegm means that older, dry blood is coming out of your baby’s nose or throat. In the drier weather, it is very easy for nasal passages to crack and bleed. There isn't a need to be concerned unless it keeps on for several consecutive days. If that happens, schedule an appointment with your doctor.
This could be due to a common cold brought on by the seasonal change and weather changes we've been experiencing! Here's a helpful link that could further diagnose the problem: http://www.webmd.com/cold-and-flu/cold-guide/common_-cold-symptoms
see a doctor