Can Phelgm Kill?

Well, not kill me exactly, but cause me to drown?

I've been training for 2+ years now. I have seasonal allergies, but they are 100% controlled by Flonase. Ever since I started training, I had the normal chlorine reactions while swimming- drippy runny nose, etc, but usually post-workout. Occasionally, I would get a decent amount of phlegm in my throat that would feel uncomfortable. No big deal- no issues in the other 2 sports, and it only happened occasionally.

Over the past 2 weeks, this phlegm seems to be waiting with reserves. On a few swims, it has made it increasingly more difficult to breath. I have no signs of a cold, no sore throat, no increase in allergies, nothing. Finally, the phlegm went in for the kill- I was doing a hard set of 100's, went up for a breath- nothing, only phlegm, I could not breath. Ok, I didn't need that bout of O2. Next breath- nothing- no air, my passage was completely blocked by this crud. My chest isn't tight, there's no phlegm in my actual lungs, I can't hack it out, my breathing feels normal- it's just blocked by the phlegm! This was the first time I have not been able to breath on my own and it scared the crap out of me.

Now, if it were just swimming, I wouldn't think it was a big deal (I would gladly stop swimming ), but lately, I've been having the phlegm creep in while running hard and biking hard. Not the same blockage as in the water, but enough to make me post this here. I have health insurance, so going to a doctor isn't a problem, but I really hate to be a hypochondriac. How can I tell if this is a cold, more severe allergies or something a little worse (like EIA?)