Unfortunately, I have has some issues with headaches with one of my children. But, she was 13 at the time and this was in 2006.
She'd had headaches almost daily for a few months and took 1 ibuprofen daily to aleviate any pain, and that worked.
The headaches started in June, and was dismissed as stress. We had alot going on; I was expecting a new baby, we were moving about 2 1/2 hours away from our home, going to a new school for 3 of them (her older sister and younger brother), possibly her sister moving to her father's house and leaving us, and she was at her grandparents for a couple weeks without me, while I was setting up our new home.
When I had all of my children with me again, her headaches stopped for a few weeks, but came back. They would come daily for a couple weeks and go away again for a couple weeks.
Things didn't work out with the new home and we decided to move back to our home area. We rented a house, baby still on the way, she was on her way back to her old school, her sister was staying with us. Things were looking good and the headaches stopped.
Again, we wrote it off as stress. When I took her for a checkup with her doctor, we did mention the headaches and he wasnt too concerned, but told me to try to monitor them and if they came back or got worse, he would send her for a CT scan.
We didn't have that chance, because when they came back, within a couple days, she woke up in terrible pain and began vomiting. I had a neighbor take her to the ER, while I went to work (2 hours), and got the new baby taken care of for the morning - she was only 3 1/2 weeks old and this occurred in November. I was also expecting the ER to take hours to even see her, much less test her and get results - I only worked 1 hour before I had someone else finish my job for the morning. I met them at the hospital and got the news.
We were told that she had a brain tumor in the cerabellum - at the base of the skull, about the size of a golf ball. We went to a Children's hospital - CHOP - and she was in surgery the next afternoon. The entire tumor was removed successfully. While she was recovering, I was told the tumor was cancerous and she would have to undergo chemotherapy and radiation therapy for 6 weeks and 6 1/2 weeks, then chemo monthly for a year. Her chemo was in pill form.
Then we got the good news. She was in remission!! Thank God! and everyone who prayed for us and helped us throughout.
She is 16 now, in high school - a junior, and still no signs of cancer.
If there is any thought in your mind that your daughter may be telling the truth about having headaches - do mention it to your pediatrician. Try to monitor them; when, where, what she is doing. I know, you stated that it usually happens in the car.
Perhaps she is just prone to motion sickness. Her doctor may suggest something for that. I used bracelets for my daughter, they had plastic pieces that put pressure on pressure points in the wrist. They worked well for her, but only when she was in the car. They didn't work for the nausea caused by chemo.
Good luck and never be afraid to ask your doctor for a scan if you think the headaches are real or if they continue.