The 'Young' singer attended the Drake gig at London's O2 Arena last night (March 27), before bumping into JLS in her private box.
"Tulisa was really looking forward to escaping all the embarrassment of the past week with a good night out," a source told The Sun.
"She knew the JLS boys were going to be there but felt really awkward when she walked in because she presumed the lads had seen the tape. She said a quiet 'hello' to them — but then went straight out the back door, let out a big squeal and took a deep breath before going back in to face them."
The insider added: "Tulisa and her pals hardly spoke to JLS throughout the gig. It was a really uncomfortable atmosphere because she is so embarrassed about the tape."
Contostavlos reportedly went backstage afterwards to meet Drake and Florence Welch, with the rapper describing her as the "Princess of the UK".
However, she is said to have left the party early after becoming concerned that people were making comments about her tape.
The singer's ex-boyfriend MC Ultra appeared in court on Monday over the tape, alleging that she persuaded him to make the film.
> Tulisa Contostavlos stepmum appalled by star over 'MC Ultra sex tape'
Watch Tulisa's 'Young' video below: