What has happened to you is really not unusual following an orgasm and will probably be experienced more than people admit.
Basically, if you actually think about what happens during an orgasm, then you may see that it is all to do with blood flow and muscular tension. An orgasm is an intense and pleasurable experience, however, when this happens there are alterations in blood flow. When you become aroused, then you get increased blood flow generally and vasoconstriction that allows the vagina to become engorged in a woman and therefore very sensitive and this also causes an errection in men. This means that a lot of your blood is going to one particular area when having intercourse. As you near climax, you will vasoconstric and your muscles will become very tense and tight and it is this that causes you to shudder. Once you relax after your orgasm, you will start to vasodilate and this causes a man to lose his erection so that blood can now increase and flow to other parts of the body more efficiently. The muscles also relax and it is this sudden vasoconstriction followed shortly by vasodilation and the increase and decrease in muscular tension that causes the headaches after orgasm. Men can also find this after ejaculation, as this is a sudden and explosive release of tension.
As long as you have no other medical problems, then I am sure that this is just an inconvenience rather than being serious at all and as I said before, I am sure that it occurs more than most people would like to admit.