I had the worst headaches, dizziness, pins and needles and the inability to speak properly when I had irritated a nerve due to neck strain - the muscles had swollen enough to pinch the nerve and, honestly, I fell like I was dying. I had a lot of tests, but nothing showed up until, out of nowhere, the GP thought 'what if it's the neck and not her head?' and referred me to physio.
I had a bit of physio, changed my pillow to one of those shaped foam ones and, after an awful first night, the second was OK, I moved my head, my neck clicked really loudly and, from that moment on, the symptoms almost disappeared, only to come back in a far milder form if I'd strained my neck again when lifting lots of things. (I also worked a lot on my posture to make sure I didn't push my head forward slightly and increase the weight on my neck muscles.
In other words, there are so many different things that can cause awful headaches (sinus problems, neck problems, posture, a bad position sleeping, tiredness, low blood sugar, stress, not eating well, vitamin deficiencies, arthritis, high blood pressure, allergies...) that horrid though it is, something dangerous is really the least likely of all.
Can I suggest you try using cold compresses if you haven't done so already? They work where a million painkillers won't work for me. Where the pain is might also give a clue - across the forehead, back of the head, over the ear and face, etc, etc. Chasing the pain around with a cold pack could give you an idea where it's the worst, for example.
Hope the GP visit went OK.