What Causes The Hair On The Head To Stand Up?

I have been doing on line research for a condition that my sister has. She is not computer savvy, and has asked me to do this. She has been very stressed and took a while to find meds that helped her, but now she has a problem with the hair on her head that stands straight up. My research showed info about the Errector Pili and how it explained that it was like having goose bumps, but those go away however the hair on her head continues to stand straight up. We are wondering if there was a medical term for this condition and what type of doctor she would need to see for this condition. I really appreciate all the help you could give me and I am thankng you in advance. Thanks