Problems with the display of lines

Since yesterday I have had problems with the display of lines. I need the lines to create two-dimensional drawings for cutting.
When I draw a line that starts from another line, this new line takes on the shape and colour of the old one and even links them together.
Where and how can I undo this so that, for example, if I draw a red line and then a blue line at right angles to it, the lines remain separate and retain their colour as before?