This is not unusual, until you find your perfect nic. level. I find it helpful to have different strengths around. Sometimes I need a higher nic level to deal with cravings, so I find that I vape different levels throughout the day. I started out with 18, which was too low for me, then got some 24 and 36. I found a 36 mg flavor that I LOVED, but got headaches from vaping it too much. Now that I have switched it up over the course of a day, I have not had any more problems.
(FYI, I have only been vaping since July 6th, and it took a couple weeks to get the strength thing sorted out, so don't worry if it takes a little time!)