RIHANNA GETS SUED BY HER PERSONAL TRAINER!

Rihanna's incredible physique is the subject of a nasty legal battle -- and it's all because the singer allegedly didn't pay her personal trainer for a few days worth of work.

The trainer -- Cindy Percival -- filed the lawsuit against Rihanna in L.A County Superior Court last week, claiming the singer owes her $26,144.14 in unpaid wages from 2009.

In the lawsuit, Cindy claims she was hired to perform "exclusive personal training services" for Rihanna for $1,500 per day -- but never got a check for a couple of weeks of work between September and October.

Cindy is also asking for $2,644.14 in travel expenses -- because she claims she had to travel all over Europe to meet up with Rihanna ... to make her bend, stretch, pump and sweat in person ... which really should be payment enough.