You are describing an intriguing medical scenario. Initially, you might have suspected that the person described might show symptoms of diabetes, given that excessive thirst and frequent urination are key symptoms of diabetes. These symptoms would have definitely made a doctor perform a blood test in order to check the blood sugar levels. You state that the glucose levels are normal, which is an indication that diabetes is less likely—however, you didn't state whether the blood test was taken whilst fasting. In order to gain reliable results for diabetes, it is important that the patient has been fasting before the blood test.
Assuming that the blood test was a fasting test, normal glucose results would make me look at other possible health issues. Bearing in mind that the patient is female, my next guess would be a urinary infection. Increased thirst and urination are the key symptoms here, too. Headaches are less frequent symptoms for urinary infections, but they can be symptomatic here, too. Therefore, you might want to do a test for cystitis as the next step.
However, there could also be a much simpler explanation. For example, if the weather is currently very warm and hot, the symptoms described could all simply be symptoms of heat stroke. Another possible diagnosis could be chronic dehydration. The patient may not have been drinking enough for a prolonged period of time. Especially if the patient has also exercised a bit more than usual in this period of time whilst not drinking enough, this could have easily led to dehydration. This would also manifest itself in headaches and increased thirst. Frequent urination would in both of these cases simply be a result of the increased consumption of water and other fluids caused by this increased thirst.