Britney Spears says a brief run-in with ex-boyfriend Justin Timberlake triggered a panic attack before her infamously bad 2007 MTV VMA performance.

Britney was set for a big return to the stage that night with her song "Gimme More" ... but was caught off guard when she saw JT at the show.

In her new memoir, "The Woman In Me," Britney recalls the interaction, writing ... "It had been a while since I'd seen him. Everything was going great in his world. He was at the top of his game in every way, and he had a lot of swagger. I was having a panic attack."

Britney says she "hated the way I looked" ... because "It was less than a year since I'd had my second baby in two years but everyone was acting like my not having six-pack abs was offensive."

What's more, Britney admits she hadn't rehearsed enough and knew the performance "was going to be bad." She says "nothing was going right" backstage ... claiming there were problems with her costume and hair extensions. She says she didn't sleep the night before, and was feeling dizzy.

Britney continued ... "I couldn’t believe I was going to have to go out onstage feeling the way I felt." She says she went out and tried her best, admitting it wasn't up to par for her.

She added ... "I could see myself on video throughout the auditorium while I performed; it was like looking at myself in a fun-house mirror."

JT, on the other hand, was on the other end of the spectrum in Britney's eyes.

She says, "Justin glided down the runway. He was flirting with girls in the audience, including one who turned around and arched her back, shaking her breasts as he sang to her."