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The 20-year-old woman who claims Justin Bieber fathered her now 4-month-old son says the pop star was “aggressive” and refused to wear a condom the night she says he got her pregnant.

“His whole demeanor changed,” Mariah Yeater told “The Insider” in an interview airing Monday night.  “It went from cute and gushy…to just more aggressive.”

Yeater’s paternity suit against Bieber, 17, claims their tryst in a backstage bathroom at the Staples Center in Los Angeles following a concert in October 2010 resulted in the birth of her son Tristyn Anthony Markhouse Yeater this July.

She told “The Insider” their sexual encounter was the first for the then-16-year-old “Never Say Never” star.

“I told him to wear a condom,” said Yeater, who was 19 the time the alleged tryst took place.  “He said, ‘no,’ that it was his first time and he wanted to feel everything.  So I went with it, in the heat of the moment, and here we are today.”

While Yeater went public, Bieber made only a passing reference to the controversy in an appearance a world away at  the MTV Europe Music Awards in Belfast.

“You may have heard a lot of crap on the Internet lately, but I’ve pulled through it,” he said while accepting awards for “Best Male” and “Best Pop Act.”

Bieber’s appearance at the awards show in Belfast also put to rest rumors that he and girlfriend Selena Gomez, who hosted the awards, had split over the allegations.  The two appeared hand-in-hand at events in Belfast over the weekend and Gomez, 19, even poked fun at the controversy in a skit with fellow popstar Katy Perry during the show.

Bieber’s team back in the U.S., meanwhile, spent the weekend hitting back hard against Yeater and the suit.  TMZ reports his lawyer, Howard Weitzman, contacted Yeater’s lawyers on Friday to discuss the suit.

The  site also reports Bieber, who claims he has never even met Yeater, will take a DNA test when he returns to the U.S. in two weeks, and may decide to sue if the results show he is not the father.

“We are going to take every step to defend and protect Justin,” Bieber spokesman Matthew Hiltzik told the New York Post.

The threat of a retaliatory lawsuit from Bieber is not be the only legal trouble facing Yeater.

Her very public allegations have placed a spotlight on her checkered past, including a domestic violence charge from an ex-boyfriend whom she earlier accused of fathering the baby she now says is Bieber’s.

Yeater faces a bench trial next month on a misdemeanor battery charge that she slapped her 18-year-old ex-boyfriend last December.  The charge could leave her facing six months in jail and a $1,000 fine.

The Los Angeles Police Department has also not ruled out the possibility they may look into Yeater’s alleged backstage tryst with Bieber as a case of unlawful sex with a minor since the singer was 16 at the time.

If convicted, the violation carries a one-year jail sentence.