Christina Aguilera had a nose job sometime between 1999 and 2000. Basically, she had the tip of her nose reduced in size and the bridge narrowed a tiny bit. The overall difference in shape is insignificant, it's just smaller now. Also, it's minor enough that you would never expect she had a nose job unless you saw her before.
The most noticeable difference in the tip of her nose.
This picture shows that Christina Aguilera's nose was wider before. Overall, it's an excellent nose job because even though it's smaller and more feminine, the overall shape and character were retained.