1. Dehydration
2. Low iron/anemia
3. Not eating properly
4. Experimenting with weight loss/energy supplements/energy drinks
5. Sick with a virus or bacterial infection.
*whoops, forgot one* 6. Pregnant.
The following questions are what the doctor is going to ask: Does she have any allergies? Take any meds, vitamins or supplements? Have any history of similar issues? When did she last eat and what? What does she think caused this?
Also, what was she doing when the symptoms started? Did they come on suddenly, or gradually?
Does anything make the symptoms better? Worse?
Describe the feelings she's having. Is there pain? What does it feel like? Is it steady, or does it come and go? Has it changed since it started?
If there is pain or discomfort, where is it the worst? Does it move or radiate anywhere else?
On a scale of 1 to 10, with 10 being the worst she's ever felt, how severe are the symptoms?
What time did this all start? Have she had this happen before? When?
Is she on birth control? Could she possibly be pregnant? Has she had intercourse?
Once you have all of this information, it will be easier to take an educated guess at what's going on. For now, the information you've provided really isn't enough.