A space headache is exactly what you think it is, a headache, but obtained while in outer space. It's recently been classified as the latest occupational hazard for astronauts. "After quizzing 17 seasoned astronauts they found more than two-thirds suffered from headaches on missions yet were headache free back on earth." (BBC)
Space headaches are unique in nature. While an "Earth headache" can be "Usually described as throbbing and worsened with movement." (Dr Kudrow) Space headaches are "Exploding," and have nothing to do with motion sickness. "The researchers believe there are a number of reasons why space travel could cause headaches, the root cause being microgravity." (BBC)
"It can also cause a shift in the body's fluid towards the brain which would raise intracranial pressure." (BBC)
Your typical headache pills did offer somewhat of a cure just like a normal headache.
Astronauts typically don't disclose all their symptoms because they're afraid of being banned from space for health issues. So this new study, done by the Dutch, is a first.