Imagine...
you just got propelled into space. This is your first ride. You are still impressed by the launch and acceleration during ascent. Felt better than take off run in a commercial passenger jet plus all the shaking and awesome noise at ignition, which was way more violent than expected and what one could imagine by just watching videos of rocket launches. So, your spacecraft just reached orbital speed. The engines cut off and from one second to the other you and everything else onboard becomes weightless. Feels a little bit like vertigo and going downhill in a roller coaster for the first few seconds, after being pressed into the seat at variable g-loads for about ten minutes...
Rationally you knew what was and is going to happen. And you’re already familiar with this due to the training you got by those parbolic flight campaigns. For now everything is fine. Big joy. Everyone is smiling. You can't even really believe you just made it into space. It got quiet, except the environmental control system, cabin fan, electronics and voice comms from mission control. You could barely gather a look through the capsules window since you are still strapped to your seat. All you can see from your angle is very bright sunlight shining into the capsule, and the blackest black you never saw in the night sky before. And you still got your helmet on, but the visor is open. You doff your helmet and gloves and now you get unstrapped, finally, after several hours.
So you move out of your seat. It's your first free float in orbit. Feels strange that zero gravity is not going to end after about 20 seconds. This time it is continous, and stays for the rest of your flight until deceleration during entry and parachute deployment. Somehow you start to feel a little bit uneasy, because you get the impression that you could "fall over". Because there is nothing below your feet and this is not going to end, unlike during parabolic flights, where you get the airplane's floor below your feet and body regularly. Every move you do causes your body to slightly move and rotate until you counteract by using your hand and/or feet to hold somewhere. It's a little bit like skating after you haven't done so for many years - a little weak-kneed. You realize you have to train your sense of balance to become used to this new kind of motions. You stretch out your arms and legs, maybe that makes this "falling over"- impression disappear. You feel almost like a mouse in a zero-g experiment.
But now it gets really weird. Your body is telling you that you are upside down, while your eyes are telling you: nope, you still got the proper orientation in the capsule. This is due to the missing weight force that counteracts gravity when you are standing or lying on the surface of the earth. Your organs are not pulled down anymore, and also your blood gets distributed differently in your body, especially in your upper body and head. It even acts on your skin, especially your face. You can feel that your veins on your neck seem a little bit thicker than usual, and also your face feels slightly puffy. So it's very obvious that zero gravity isn't only acting on your body but even inside you entire organism.
You now do your first backflip; a half one. Now your eyes are telling you that you are upside down in the capsule. Your crew-mates are all oriented differently while moving around. One next to you is also upside down, in relation to the capsule. You look at him/her and it seems very strange that he/she is not upside down in relation to you, but you both are upside down in the capsule together. And no matter how you re-orient your body in the capsule, the physiological impression in your body of being upside down just doesn't disappear. And every new orientation via a flip also causes a slight vertigo for a fraction of a second. Uuuuh. You do realize that it might take a while to get used to this new crazy environment which actually has no up and down at all. And that's the point where you start to become a little bit uncomfortable. A slight cold sweat, and a slight nausea which hopefully doesn't get too strong... ?
Damn! And you thought you would be a real man that can handle it for sure, of course ? Although rationally you knew that anyone can suffer form it ?
I remember the second and last flight of German Astronaut Hans Schlegel, STS-122. He obviously got really sick. He consulted the flight surgeon a few times afair. I think that's why his wife chosed the German song "Maenner" (men) for the wakeup call on FD4.