Most of the folks that go directly into their lungs use little or no nic.
^This. I lung inhale now, but I also vape 2 mg or zero mg nic. I could never do that with 18 or 12. So switch to Mouth-To-Lung style for now.
Also, I had some weird experiences with e-juice when I first began - just bought whatever they had at a local smoke shop (not even a vape shop - just a smoke shop with a vape counter and two brands of juice). The juice was not good and I had an adverse reaction. So definitely try some other juices - Mount Baker sells very reasonably priced juice and you can customize the ratio of PG to VG. It's a good place to start when you are experimenting to see what works for you (without spending too much). You can move up to the more expensive brands once you find your sweet spot.
These days I prefer 100% VG or close to it - it produces a lot more vapor, so I've learned to prefer it. But also moving in that direction may help with what you're experiencing, as PG allergies are fairly common
Another thing to do is drop nic level. When I first started vaping, a lot of people recommended 18 and 12 mg nic for me based on my previous cigarette type and frequency of smoking. But it really was too strong for me. I found a comfort zone with 6 mg for a long time. Something to keep in mind. You'll know if the nic is too strong if you feel dizzy, headachy or have trouble sleeping.
Hang in there and keep on vapin'!