A clip of Contostavlos performing a sex act on MC Ultra - real name Justin Edwards - leaked last month, prompting the singer to obtain a gagging order.
In an eight-page witness statement to the High Court to extend the injunction, she confessed that she couldn't remember making the video, which is why she denied its existence last year.
She also admitted that the leaking of the tape, which was filmed backstage at an N-Dubz gig, has had a "severe" impact on her health.
"I believe I must have been intoxicated at the time the video footage was taken. Now that I can see the video, I can see it is me in it. I must have been very drunk to let my then boyfriend Justin Edwards, who I also know features in the video, take the footage," The Sun reports the statement read.
"Justin Edwards claims in a letter to his solicitors that I 'took many pictures and videos of us having sex' and I still have those videos. That is completely untrue. The commentary on the website says that the video was taken in a public place.
"I can say for certain that it was not. It must have been in a private dressing room or toilet in a dressing room. My dressing room always had the door locked and there was security outside. It looks like I was at an N-Dubz gig."
She continued: "I was unsure whether it really existed. I was, however, suspicious of Justin Edwards. He admitted taking footage of me on his mobile but said he deleted it before he sold the phone.
"I had, and still have, no recollection of such footage being taken. He became uncomfortable when I asked what the footage was. I was scared someone might have it."
Contostavlos added that she suspects Edwards made other films of her. She also revealed how she was told by his ex Nisa that she and others had seen the tape. "She said he used to say he would wait until I became big, then release it to make money.
"I believe Justin Edwards is a manipulating and bitter man. He has a jealous vendetta against me. This video is part of a plan to damage my life and career."
The singer also thanked X Factor boss Simon Cowell for his support. He recently offered her £500,000 to return to the ITV1 show as a judge this year.
Contostavlos is suing Edwards for £100,000. He denies leaking the tape and a trial is due to go ahead in summer.
Watch Tulisa Contostavlos discuss her sex tape below: