there is not enough information to solve the problem without going into speculations.
1. Try posting code you think might be the culprit.
2. Add more information to the problem. Try to be as descriptive as possible.
3. Explain how your game work. What is normally supposed to happen when you move the game window?
4. Do you use other library or engin?
Since you mention SDL, i assume this is a simple 2D game. What kind of physic do you use?
As you can see, more information is needed.