Dairy absolutely makes me phlegm-y, it's gross. I too used to be very prone to sinus infections, and have sensitive sinuses in general. Cutting out dairy when I started PB eliminated any problems with my sinuses almost entirely...then a hunk of aged gouda got the better of me. I've been on a dairy streak for about 4 weeks, including cheese and heavy cream (no milk/yogurt, milk upsets my stomach immediately), and have woken up with phlegm in my throat/a dry and sore throat many days per week.
I know it's from the dairy because nothing else has changed in the past 6 months since starting PB...so I'll have to go back to really good dairy as a very occassional treat instead of as an everyday salad/dinner topping. Oh well, once I cut it out I don't miss it after a couple of weeks and my throat feels better.