I'm going through the exact same thing. I've had a headache for about a month now. I've found out I'm allergic to PG, I get rashes and itchy skin and feel horrible if I take PG. I'm now on 100% VG which helped that but I still have the persistent headache.
I didn't want to believe it was electronic cigarettes doing it. At first I thought it's possible I'm ODing on nic because I was on 18mg juice which seemed high at first but after reading the research done that indicated around a 10% absorption rate, I should really be getting about 1.8mg of nic a day because I vape about 1ml. 1.8mg? That's one cigarette's worth a day.
I dropped down to 6mg juice to be sure that it's not nic related. So if I follow the 10% rule, 0.6mg, about half a cig a day. Yesterday I vaped almost nothing because I felt really ill and wow today I had the worst headache I've had in a really long time. I could hardly see straight it was that bad.
I've ordered 24mg juice that should be here in two days to find out if it makes a difference. I've been vaping for over 6 months now, mostly on 18mg with occasional headaches which I think was related to the PG allergy.
I have been to the doctors about it. My bloodwork came back fine, blood pressure fine, nothing out of the ordinary they could tell me. It might not be the same for you but for me I can only assume either any nic at all doesn't agree with me or I'm going through nic withdrawl. Judging by the 10% absorption it seems very likely. I used to smoke 10 - 20 rollies a day for comparison.
Just what's happened to me and where I'm going with it and hope you get some relief yourself because I know it sucks.
All I know, is if I can't vape, I'm definately not going back to smoking because it destroyed my health but everyone's different. I'm young enough to bounce back and hope I can recover fully with vaping. If I get relief from higher nic I'll let you know. Feel like a guinneapig but all in the name of science and goodwill!