When I start my pc, the first screen I see, has a small resolution,
Yes, this is normal. Most IBM compatibles desktop PC'seither boot in text mode our use a low resolution graphics mode which just about every monitor from the last few decades can handle. This is good.
but by the time I get to the login screen, the resolution is good.
Your operating system will load drivers for various devices, including drives for your graphics card. That enables more than just the low resolution modes. Typically it will also remember your settings from the last time and use those. This is why it changes resolution once the OS is active, but not before it is loaded.
Given these two points I am convinced that your resulution never was any different in the past. However that does not mean that the image on the screen is not different.
A low resulution image on a higer resolution display can be treated in several ways, e.g.
- The image can be drawn only in the top-left corner.
- The image can de draw centered in the screen with black bars around it.
- or, the image can be stretched. That way it still fills the screen, even though it is at a lower resolution.
The last can be done both by the graphics can or via the electronics in your monitor. I suspect that you changed something here. Check these settings.