AutoCAD 2010 :: Lines Not Smooth On Work Area?

while tracing a plan on Autocad 2010, by accident I clicked on something, or performed some unknown commands. My lines, that were smooth just seconds before, suddenly became really pixelated, as if made of many small segments. And when rendering the plan on pdf, it is still the same, and the lines are really thick. I have checked, the line thickness is on default (0.25mm), but compared to renderings I've done before, this time the drawing has really awful lines.