It is known that some people get splitting headaches during or after sex. One cause of headaches in muscular tension in the neck. Since muscles tighten throughout the body during sexual arousal, it is possible that this is what is triggering your headache.
Since you weren't reacting this way in the past, I have to wonder what has changed. Are more tense these days? If this is true, then your neck may be already tight when you begin having sex; in which case, the added tension associated with sexual arousal may be just enough to push you over the edge and cause a full-blown headache.
The fact that your head hurts on one side makes me wonder if you are experiencing a vascular or migraine headache, which is associated with changes in the blood vessels.
Migraine headaches occur when the blood vessels constrict and then dilate. Since sexual arousal does increase blood pressure and cause vascular dilation, you may be developing a migraine headache because of this.
There are lots of natural ways to treat migraine headaches including the use of feverfew herb and relaxation exercises. But, before you look into alternative therapies, you should see your doctor to make sure that you are all right.
Once you are told that no serious health problems exist, then you can begin a multi-pronged approach to the problem in which you work on the emotional and physical causes of the headache. Address what is getting you worked up and take steps to resolve what is troubling you; if your headache is due to muscular tension, a visit to the physical therapist can help you learn how to relax the muscles; also learning relaxation exercises, meditation techniques, and/ or yoga may help. You may also want to take natural remedies, including feverfew herb and minerals such as magnesium, to prevent and/or ease the headache. See my Wellness Dictionary for a complete list of alternative approaches.