Do you have any neck and shoulder pain? I have two bulging disc in my cervical neck, which causes terrible headaches on the right side of me head. If the headache keeps up, I would see a doc and ask for a MRI of the neck/head.
It sounds like tension headache to me too. The best thing I've found to get rid of one of those is to use the extra strength migraine relief advil (though these are very strong, and only would recommend them if the normal advil don't cut it for you.. for me these are the only OTC that give me any relief), get a cool cloth for your head and get some rest.. also need to make sure your well hydrated as that's just going to make it worse if you're lacking fluids.