EDIT: 12/2019 Since this question still occasionally gets hits:
In the years since I asked this question I found out that I have sleep apnea. I also learned about the effects of mouth breathing which significantly increases your risk for a variety of problems, such as this one. I've been doing myo-functional therapy exercises (with this therapist), learning to play the Didgeridoo, and reading and practicing the exercises from the book "The Oxygen Advantage". I also now sleep with mouth tape. I've still got a ways to go, but my journey in learning to breath better has improved this quite a bit.
I have a bit of an odd question, whenever I do cardio workouts that get my heart rate up and breathing hard I get phlegm buildup in my throat and my nose starts running. It's not a huge amount but it's enough that I wind up spitting and wiping my nose frequently during workouts (the snot sometimes comes out greenish but mostly clear).
This has been happening pretty consistently for as long as I can remember. I regularly run, bike, swim, and do pilates (typically 2 - 3 times a week for each though I've been falling off lately with pilates). Pilates is the only exercise that doesn't cause this because it's a slower pace more focused on core strength and flexibility so my heart rate doesn't get very high and I'm never pushed to breathing heavily. It's also not effected by workout frequency, the times in my life where I've been working out less/not at all have not decreased this when I start up again.
Does anyone know what would cause this and what I could do about it?