Could be few things, like some kind of allergy from the juice flavouring that thus juice u vape contains or more rarely an allergy on the propylenoglycol that ais one of the main ingredients of the juices but also i gave heard that it can be caused from the material of the coil itself.
From my personal experience vaping the last 2years or something i only once had this problem when a vape atomizer i used was spitting juice a.k.a spitback.
This happens when the coil absorbs more juice than it can vaporise and it means its not working good either because of using the wrong vg/pg juice percentage for the vape device u use or because the coil itself or the vape device is just bad.
If u are careful u can notice if u have spitback its like small drops of juice hitting your mouth, sometimes they are hot so its easy to notice but others are not hot and they are very tiny snd its not easy.
I actually use my tongue when i want to see if there is spitbackbecause its easier to notice it, while inhale i just make sure the tip of my tongue is on the way of the air as it comes out of the driptip so any spitback will hit it and i notine immediately.
I just said the things i know but i cant know why in your case it happens just saying so u have in mind and i am sure u will figure it out