I have the same issue. I seem to think it is related to the vaccine. I never had this thick phlegm in the back of my throat until after I took the vaccine. I had Covid in 2019 and 2020 and did not take the vaccine until 2021 when I was going to go on mission trip out of country. My sister-in-law has the same phlegm problem. She is in health care and was mandated to take vaccine. Never had problem until after shot. My grandson who is cross country runner now has POTS. Dr. Says it is from covid. I believe it is from vaccine. Has there been any research done on the side effects of mrna vaccines?