Ke$ha -- GREAT ASS ... Or Optical Illusion?!!?

Holy crap ... Ke$ha just posted some racy new lingerie pics ... and she looks INCREDIBLE!!!

Butt ... did the pop star really tone up her body -- or are the new pics the result of some creative camera work?

Here's the deal -- Ke$ha posted the pics on her Instagram page yesterday, which feature the singer showing off her G-string'd ass in various sexual poses.

In the pics, Kesh looks WAYYYY different than she did strolling the beach back in 2011 (see below).