Going into space or experiencing weightlessness is incredibly strange and our bodies are not used to it. We evolved over millions of years with gravity and taking that away is a big shock to our system.
We know that when astronauts first arrive in space they feel pretty rough and it takes about 24 hours to get used to it and feel relatively normal again. This process is reversed when returning.
The balancing system in our ears adapts to the lack of gravity so struggles to make sense of everything back on Earth again. It is for this reason astronauts who have just come back from space are told not to drive their cars for some time before they get used to being on Earth again!