everyone is different, there are those that are intolerant to VG, some PG. Try some 70% VG and see if it gets worse, than compare to 70% PG juice.
Personally find PG to harsh, but that's me.
The other thing that could be going on is if one has had a recent cold or lung inflammation, and the heavy vaping is just cleaning out some damaged air sacks. Just got over a very nasty virus, and between vaping and the virus, opened up some damaged lung area, was getting a lot of hacking and nasty hackups, but as I recovered, severity dropped and everything is back to normal, but with increased lung capacity, or something like that.
Find that the longer one vapes, occasionally will get some additional cleaning and repair symptoms, as the body continues to heal itself.
Vape long and Prosper.!!!