What am I doing wrong, trying to draw with paintbrush and will only make straight line

This makes no sense and the first time this has ever happened to me... I've got a setting wrong somewhere but have no idea. I select the brush tool and try to draw... nothing comes out until I lift the brush and then a straight line shows between where I started and where I ended... I'm dumbfounded.... How do I set things right again for normal drawing and painting? It's the straight soft line brush stroke in this drawing I'm talking about. The rest of the drawing is a scan of a pencil drawing so not a part of this question.

TIA,

Ken