It may or may not work for you but I sit in an upright chair and massage the back of my neck, then my back as far down the side of the spine as your arm will allow (right arm stretched across your front and massage the back left and visa versa)
Then, stretch you arms as far as up as you can as if reaching for something - hold for a few seconds then, keeping them fully stretched, slowly start to bring them down to your sides then relax. Repeat this 3 or 4 times.
Then stretch your arm out in front of you again as if reaching for something, slowly bring them down to your knees but still outstretched, then relax. I repeat this a few times. It does sound like what I call a 'tension headache' and i fully relate to the "being hit on the back of the head with a baseball bat". This will help release the muscles that are causing it.
It works for me 90% of the time. I really hope it starts to give you some relief soon cuppa. So many wouldn't have gone into work in the first place so, if you really can't stand it, you must go home. I'm sure the delightful (not) Shakira will be only too happy to cover for you.
pc
Sex ~ Its the only cure.
:-}