Thanks for writing to us with your health concern.
Firstly you immersed your head in ice cold water. Immediately that will cause pain since the body homeostasis is disturbed. Normally the body's temperature is around 37 degrees, so when you immersed your head, the head could not adjust to the new tempature, and there was probably some amount of vasoconstriction, leading to a decreased supply of blood and oxygen to the brain. I will adivse you not to repeat this kind of an act again in the future. Moreover you were also holding your breath, that also leads to reduced blood supply to the brain and other vital organs. Normally we have around 500 ml of gas left in the lungs, which is reserve and once that is used up, there is a reduction in the oxygen level in our blood,and it can cause slight amount of pain, in the peripheries or even the chest. Brain is no exception and hence that could have caused a reduced blood flow to your brain at that point.
One more thing that can cause your headache, is your inner ear. The inner ear is very sensitive to temperature changes, and anything sudden like that can lead to sudden vertigo, dizziness and headache.
So as of now there is nothing to worry. In case there is some minor irritation in your inner ear lining, it will heal by itself. So give it another 3-4 days, and kindly avoid this kind of an act again. It can be dangerous and you can even lose consciousness. I understand its your job, but health comes first. Doesn't it?
For the time being just take Paracetamol twice a day for the next 3-4 days. If any other symptoms develop, do write to us immediately. We will assist you further,