Pain Behind Eyes and vomiting?

So last night, my sister randomly started crying and saying her eye hurt...really bad. On a scale of the pain from 1-10 she said it was a 10, and she thought she needed to go to the hospital. Then, she started projectile vomiting about 6 times in a minute. So my mom drove her to the hospital, and when we were almost there, she said the pain was totally gone and she felt fine. She even felt good enough to go to school today, and soccer practice. We took her to the doctor earlier, and she said she has no idea what it could be, but it was probably something called a cluster headache. Does this seem right to you? What is a cluster headache?