It's a wrap for Nikki Bella in the WWE ... the former champ says she's closing the book on her wrestling career -- claiming her body just can't handle the punishment anymore.
Bella said she realized she needed to retire after returning from a recent tour or Europe -- saying, "[The tour] was good but I just feel like I'm too old for that travel."
"The travel was really really rough," Bella said on her reality show, "Total Bellas" ... "So, I realized, I was like, why am I doing this?