What is the biological reason (what really happens) when someone gets an ice cream headache?

Hope this helps!: http://psychology.about.com/b/2010/07/26/why-do-you-get-brain-freeze.htm this should be helpful!

Ice cream headaches: Ice cream headaches are caused by cold material moving across the roof of your mouth and the back of your throat, as happens when you eat ice cream quickly or gulp a cold drink. Scientists are still unsure about the exact mechanism that causes this pain.