cannot see my selected lines in Illustrator cc

Hi

I'm just trying to do the same job I've been doing for years but have a sudden problem seeing which lines I've selected. I suspect I've switched something on or off somewhere. I select a single line and it is highlighted. I tselect more than one and there's just a great box which also encompasses lines I was not selecting. The individual lines are not highlighted (as they used to be) so I can't see what really has been selected.

Can anyone shed any light on this?

many thanks

Amanda