Everybody who took part in answering your question had such great answers. Each one could be right. WonderingMarvel is correct about you possibly having a migraine. Kingko is right about caffeine causing migraines, especially in large quantities. He also mentioned that each person is different, as did Cosmicriver12. Cosmic also mentioned that it could be overuse of caffeine and the need to cut back, an underlying condition or even another ingredient. Magicgoon22 inquired that it could be tyramine sensitivity/poisening. Although, it is rare for tyramine sensitivity to come from just one source of chocolate etc. it is still possible. Albeit, each one could be right, there is another possibility that nobody has mentioned. That possibility is an allergic reaction. Not all allergic reactions come with hives, watery, itchy eyes, or anaphylactic shock. Sometimes just a simple little headache can be your only sign. I know because I had an allergic reaction to a medicine as a kid, and all I had was a headache. It wouldn't hurt to go to the doctor, just in case. Any one of these people on here, including me, could be right. It is always better safe than sorry. Good luck.
There is caffeine in hot chocolate. Caffeine is known to help stop migraines, as well as other types of headaches. It is in several different types of headache medicines, including over the counter Excedrin and by prescription only Fioricet. Granting all this, caffeine can actually cause headaches and migraines as well. It depends on the individual, the reaction one has to caffeine, and the dosage of caffeine one ingests. If you are over indulging, I would try cutting back. If you don't use a lot of caffeine and it continues, even after cutting back, I would discuss it with the doc. Especially if it is bothersome and happens every time, or nearly every time. It may be a sign of an underlying condition or problem. It may not even be the caffeine. It could be another ingredient in the hot chocolate causing your discomfort. If you have insurance and are able to go to the doctor, I suggest going. If you cannot go to the doctor for any reason, I suggest you stop, or at least cut back on drinking hot chocolate, as well as other caffeinated beverages. Good luck.