Everytime i eat pork i get a headache, why does this happen?

Depends: Is it fresh pork or packaged pork? This often happens with preservatives that are added to it. If it causes headache you should probably avoid.

Food Additives: A number of food additives like msg, nitrates, and even some artificial sweeteners can cause headache. Also, heavily salted foods might cause headache in some situations. Also, foods that are pickled like sauer kraut, pickles obviously, chocolate and some red wines can cause headache.You might want to keep a food diary where you write down what you have eaten and when your headaches occur.

Allergy maybe : If you don't get a similar reaction with other foods, then probably an allergy---best to avoid pork, then see an allergist for testing.