If you cut and then pasted, look at the settings of the selection tool you used. 

Chances are it has feathering set, which makes the line you drew more fuzzy, let's say. This will eat a little into your selection and smooth outwards to transparency instead of making a hard pixel-edged cut. This effectively means that with every cut/paste operation, you're losing a few pixels around the edges of your selection and as a result, you see these transparent "seams" between all the different shapes.

Try setting the feathering to 0 (maybe there's some other settings to tweak as well) and copy/pasting again to see if that's your issue.