It's rumored that before hitting it big, Demi Lovato had a nose job. The plastic surgery rumors were started because of some before and after pictures of her in high school. If we take a look at this first photo, there's a couple things to consider. In the before picture, the overexposure of light accentuates her nose which makes it look bigger. In the after photo, she's using some contouring on her nose which makes it look smoother. While the before and after picture is a little misleading, there's still an argument to be made that she had a minor rhinoplasty though.
Seeing anyone with and without makeup will make it seem like they had plastic surgery. Demi Lovato is no different.
It's really up to you to decide if Demi Lovato had a nose job or not. It's tricky because her nose looks a little big in both before and after pictures (which isn't a bad thing! It fits her face.).