This is how the flight was described in the New York Times, October 6th, 1961:

Major Gherman Titov, the Soviet astronaut, felt sick during much of his 25 hour trip through space last August, two Russian scientists disclosed today.

The unpleasant sensation was attributed to the prolonged period of weightlessness during Major Titov’s 17 orbits around the earth.

The report aroused considerable interest among United States space medicine specialists, for a bore on one of the central and still unanswered medical problems of manned space flight. This is the question of how long man can endure and function under the conditions of weightlessness that result when the motion of a spaceship offsets gravitational forces.

The question bothering bio astronautical specialists, however, is whether gravity oriented man will be equally unmoved by weightlessness on a sustained space flight, such as a weeklong round trip to the moon.

In this connection, the reactions of Major Titov were of particular significance because he has endured weightlessness longer than any other man by being in a zero gravity state for about 24 hours in his 25 hour flight.

During weightlessness, the report said, Major Titov noted “unpleasant sensations of the vestibular character“ that were "felt stronger and stronger, especially when the astronaut sharply turned his head or was observing swiftly moving objects."

The feeling of weightlessness can be removed, the Russians pointed out, by creating artificial gravity in the spaceship. Whether this is absolutely necessary and how much artificial gravity will be needed will require further experimentation, they said.

(Via Weightless State Made Titov Sick)

As far as I know, the first person to use the phrase "space-sick" was Hugo Gernsback in Ralph 124c 41 +. He wrote about it in more detail in an expanded edition of the book published in 1929:

Ralph grew more despondent each day, and his hope of bringing his betrothed back to life grew dimmer and dimmer as the hours rolled on. For the first time since he left the Earth he became space-sick. Space-sickness is one of the most unpleasant sensations that a human being can experience. Not all are subject to it, and it does not last longer than forty-eight hours, after which it never recurs.

On Earth, gravitational action to a certain degree exerts a certain pull on the brain. Out in space, with its practically no gravitational action, this pull ceases. When this happens, the brain is no longer subjected to the accustomed pull, and it expands slightly in all directions, just as a balloon loses its pear shape and becomes round when an aeronaut cuts loose, to drop down with his parachute. The effect on the brain results in space-sickness, the first symptoms being violent melancholy and depression followed by a terrible and heart-rending longing for Earth.

During this stage, at which the patient undergoes great mental suffering, the optical nerves usually become affected and everything appears upside down, as if the sufferer were looking through a lens. It becomes necessary to take large doses of Siltagol, otherwise brain fever may develop.