I say this when something happens that feels scary or creepy happens, and then feel fear or adrenaline because of it. "The hair on the back of my neck stood up, because I saw something move in the forest, and I felt like I was being watched." "I was walking in the cemetery last night and the hair on the back of my neck stood u."

When adrenaline kicks in. This could be good or bad. You could be watching a horror movie, or riding on a rollercoaster.