You probably aren't hydrating enough. I find that if I don't, things that cause Phlegm: pollen, other allergens, a slight cold, etc. are exacerbated by being dehydrated. There is a slight chance you are having a negative reaction to the PG (or VG for that matter) so you might want to try 100% (or as close as you can get) to both and see how you react. Personally, I tend to vape high VG juices as I find they dry me out less and I have less of a problem with Phlegm, etc. When I don't pay attention to getting properly hydrated, particularly with a PG juice, I get the *worst* post nasal phlegm that is pretty awful to deal with. Vaping dries you out like crazy - drink a lot of water.
Also, if you are NEW to vaping, you may be clearing out all the old "bad stuff" from smoking. My wife and I both went through a period of adjustment that lasted several weeks as we cleared our systems. Hacked up lots of lovelies... but this does, thankfully, end.