when I put my game into full screen it begins to lag. I have not changed any of the default settings, please tell me whats wrong

What screen resolution and graphics card model do you have? Which Godot version and rendering method are you using? Is this a 2D or 3D project?

Laptops that have high-resolution screens and integrated graphics will generally not be able to run 3D smoothly in fullscreen without some form of resolution scaling.

If it's a lower end device, fullscreen can be a lot more pixels (1080p is twice the pixels of 720p, so roughly half the performance).