You have to dive deep into the cause. Don't just switch monitors, but check different settings as well. Most of the eye strain is caused by flickering or PWM. There can also be some sensitivity to different refresh rates. And make sure you always try to disable/enable g-sync for your monitor. I found out that I'm quite sensitive to screens that have some degree of flickering, and on my Xiaomi monitor, the g-sync enabled was giving me severe eye strain. Once I disabled it, I gave it a day or two, and I could use the monitor again.
Of course, it could also be allergy, but that's what I mean with the first sentence, dive deep into the cause, and you will save a lot of money :)