It's your neck.
I'm willing to bet that you push your head forward on a day to day basis, thus creating a massive amount of force on your cervical vertebrae, creating the pain at the insertion points on your skull. The dizziness and nausea is likely to be from the strain and tension in the sternocleidomastoid - the muscles that run along the side of your neck to around the back and top of your ears.
Dealing with it:
Immediately - Ice packs. They are a lifesaver.
Have a bath where you attempt to lengthen your neck whilst resting your head against the internal back of the bath - the idea is to try to keep your shoulders as far away from the back of your head as possible. You may have some clicks.
Long term:
Sort your posture out.
An S shaped foam pillow keeps your head and neck aligned, rather than pushed forward on high pillows as you sleep. Mine was incredibly uncomfortable the first night, after the second night, I moved my head slightly, all the joints cracked and the pain almost disappeared.
Gentle, controlled head and neck movements to lengthen the muscles - you might need to do these laying on the floor at first to support your head. Slowly tilt towards each shoulder in turn to feel a gentle stretch. Don't force it, just move until stretching, not pain. Return to the centre each time. Slowly turn your head as though to look along your shoulder and return to centre. Wiggle your shoulders as though you're increasing the distance between them and the back of your head.
As this becomes easier, you can do it sitting up or, at first, standing against a wall.
You can, if you wish, apply a little pinching pressure to the points on your neck where the most tightness appears to come from as you do this.
Check you aren't rounding your shoulders forward - likely with the rubbish non adjustable height on most buggies. Lengthen your neck and move your arms - for want of a better description, out like a T, palms up, trying to make your upper back as flat as the floor.
Then slowly bring them out front like a cartoon Frankenstein's Monster, palms down. If you get a light, fizzy pins and needles feeling in your hand, it's tight muscles across the bit from neck to shoulder restricting - flick your fingers out repeatedly (think actions from Twinkle Twinkle Little Star of going Ka-Pow) and slowly return to the T position.
At the risk of sounding more of a wally than I do already, balancing a book on your head and walking with it really helps you get the idea of where your head and neck should be.
Maintenance.
Keep up those stretches. Use cold packs. Try to stand tall. NO bending your head forward to use your phone - hold your phone up. Make sure your computer screen is as near to directly level with your eyes as possible, boosting it up with books works.
No slumping on the settee or chairs. You need to reset your posture like a Ballet Dancer.
When the world resembles some form of normality, Pilates is the best exercise to keep you functioning well. Yoga is the next.
Been there. Had MRIs, physio, a fuckton of painkillers. What worked was the above. Posture, posture, posture.