Airflow, sounds stupid I know but experienced it myself. Build some coils, wicked it, same juices as usual. Started at 15 w moved my way up, but even at low wattage I could feel a harsh hit back in my throat. While increasing the watt made it just worse. Checked my ohms but nothing were fluctuation, checked screws/posts/510 connections, the wicking itself etc. Nothing seem to be off, yet I got this horrible burn. I couldn't figure out why, because my other rta's/RDTA don't do it. I didn't change nicotine or juice flavors/brands. Turns out, I had to open/close the airflow a bit, all the sudden the throat hit was gone, even when adjusting or not adjusting the wattage to lower/higher.
Also during vaping, the wire will expand and depending on material, some do more then others. Have to check the screws/connectors from time to time. Same with super sweet juices that gunk up coils at a quicker rate, some of that can have that effect as well, but sounded to me, it was with fresh coils, hence that isn't the issue here, most likely.