That would be because you're rotating the sprite. A way to fix that would be to click on the button that looks like [<->] on top of the scripts pane, in the same pane where all the sprite's information like X and Y position and name are kept.
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To add to what coolstuff and ScratchReallyROCKS said:
The top button gives full rotation of the sprite.
The middle button only lets the sprite face right or left.
The bottom button 'locks' the sprite so it cannot rotate.
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