Last xray found 6mm lung nodule but it was not on an xray fromvthe mpnth before. why so sudden? ciuld it be a mistake?

Any number of things: could mimic a lung "nodule" including artifact, nipple shadow, blood vessels, etc, which, even though it is not a "mistake" could be something inconsequential. It could have been there on the previous x-ray, just not detected. Have your doctor who ordered the study look at it to see if another x-ray or imaging study is warranted.

See below: Lung nodules are much harder to see on CXR, especially one so small. My guess is it was there, just not visible on CXR.

A chest xray: is not the best test for finding small lung nodules. Sometimes, due to technical factors, they may be difficult or impossible to see. Other times, normal or superimposed structures(e.g. blood vessels) may mimic a lung nodule. A CT scan is the most accurate way to detect and evaluate lung nodules.