What your homeopath has done is chosen the remedy, Glonoine, based on the law of similars. If a remedy can not cause the symptoms you are already experiencing, it can't cure them, either. Your vital force has reacted by exciting your original symptoms, and making them worse. In a very short time, your headache should begin to abate, and in no time at all, it should be gone, completely. This example is a very simplistic illustration.
Homeopathic aggravations are usually much more complicated and complex.
But, they are a good sign, as far as a cure being imminent. A modality aggravation is not a symptom, but, rather, an individual qualification. Modality aggravations describe modifications of a symptom, or the whole patient, towards a worsening of their condition. Homeopaths use modality aggravations to influence the best choice between several remedies. For example, even though several remedies may be strongly indicated in your case, a modality aggravation specific to a certain remedy may point your homeopath in that direction.