First let me say I'm no doctor.
That said headaches have many causes. One of which is dehydration. I would check to make sure that she has a high enough intake of water. I would say that anything less than 3 liters a day is not enough. Depending on climate and time of year and lifestyle it could be much more. For example I'm pretty active, and its already starting to get humid down here--so I've had to bump my water intake up to 4.5L per day. I may be having to drink as much as 6L in ultra-summer.
An other factor could be nicotine. It could be too high...or it could be too low. The funny thing is that overdose has the same symptoms as withdrawal.
Also has she dramatically reduced her intake of caffeine as well? I noticed that when I started vaping I reduced my caffeine intake as well hand had caffeine withdraws to contend with.
Also there are environmental factors such as seasonal allergies, stress at work and so on that could be causeing these headaches.
Assuming you can rule out all of the other variables I would then look at nicotine strengths.
over dose symptoms but that is at about 60mg.
Vomiting and nausea, ........
Headaches
Fainting
Difficulty breathing
Pallor
Sweating
Palpitations
Lisps
Stomach pains/cramps/bloating
Seizures
Weakness
Drooling
You may notice symptoms of nicotine withdrawal within 2 - 3 hours after you last use tobacco. People who smoked the longest or smoked a greater number of cigarettes each day are more likely to have withdrawal symptoms. For those who are quitting, symptoms will peak about 2 - 3 days later. Common symptoms include:
•An intense craving for nicotine
•Anxiety
•Depression
•Drowsiness or trouble sleeping, as well as bad dreams and nightmares
•Feeling tense, restless, or frustrated
•Headaches
•Increased appetite and weight gain
•Problems concentrating
Nicotine use can have many different effects on the body:
•Decreases the appetite (for this reason, the fear of weight gain affects some people's willingness to stop smoking).
•Boosts mood and may even relieve minor depression. Many people will feel a sense of well-being.
•Increases activity of your intestines
•Creates more saliva and phlegm.
•Increases heart rate by around 10 to 20 beats per minute.
•Increases blood pressure by 5 to 10 mmHg.
•May cause sweating, nausea, and .........
•Stimulates memory and alertness. People who use tobacco often depend on it to help them accomplish certain tasks and perform well.
so really not the same with overdose to underuse.