Why do some single pixel lines become blurred

[Gigatronix Pete]

Hello team

I'm doing some very small vector work & for some reason all of a sudden I can't draw a single pixel straight vertical or horizontal line clearly. 
Whenever i try i comes our blurry like this ...see attached image highlighted in red.

Any ideas?

 

[GarryP]

My initial guess is that you are drawing a rectangle which has a 0.1pt stroke aligned to the centre which would mean that the software is trying to infer the pixels around and inside the rectangle. If you need to draw a rectangle to create a line then I would advise using a Fill rather than a Stroke.

[Gigatronix Pete]

Hey @GarryP thanks for the reply & yes i do use a rectangle for single pixel lines but do also always use the fill as the colour.
99% of the time it works fine but every now & then this happens  ...all i do each time is cut the main parts of each image out into a new document & start again  ...then it's usually ok.
But this time it kindahappened halfway through so i thought i'd ask in case i'd pressed something by mistake