I got the same results, except I was doing dumbell flies. I went to the nurse and she told me that I had pulled a nerve in my neck, and gave me robaxacet for it. It worked wonders. I went back to doing dumbell flies, just restricting the range of motion at the bottom since I was putting unncessary stress on my neck from extending too far.
For your own interest, the "pain" I got was a sharp headache at the back of my skull, near the "bump" at the bottom, not sure of the name. It felt like a sledgehammer blow as you've said, or a dull migrane.
Though, it could always be extended pressure on the head from not breathing properly. You need to be very careful to breath right when weightlifting. If you don't, the results can be catastrophic.
Sorry to hear about the MRI repayments. At least you know you'll still be alive to pay them!
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