Pixellated drawn edges

Hello,

Part of my work involves draweing in Photoshop. Everything has worked just fine. However suddenly I'm getting pixellated edges on my drawn edges. Usually I draw withnthe pen tool then convert to a selection and fll with colour. For comparison in the image attacehd you can see the old edges, which are good, on the black shape, and the nedw, bad effect on the yellow shape. This is also happening with selection tools. But if I make a shape and concert to a selction its still good.

Any help gratefully received! Many thanks

Richard