Circle tool stuck making ovals in Illustrator

I've done something to my shape tool - it's only producing ovals instead of circles, in spite of holding the shift key down. I've found that it's also making parallelograms out of squares - skewing the objects.

I've tried to get a screen grab for you showing the cursor - as this looks different, but Snagit isn't having it, so what I have done is mocked up the cursor in Photoshop. Here it is below: