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Christina Aguilera, Matt Rutler

Steve Jennings/Getty Images for Breakthrough Prize

It didn't take Christina Aguilera much time to lose those pregnancy pounds!

On Sunday, less than three months after giving birth to daughter Summer Rain, the 33-year-old mother of two showed off her incredible post-baby body in a plunging red Rubin Singer dress. Attending the Breakthrough Prize Awards Sunday in Mountain View, Calif., with fiancé Matt Rutler by her side, Christina looked teeny tiny in her sexy gown, which perfectly accentuated her svelte, trim figure.

So how on earth does this hot mama, who also has a 6-year-old son, Max Bratman, with ex-hubby Jordan Bratman, look this great  so soon after welcoming baby No. 2?! A source tells E! News the singer "was very healthy throughout her pregnancy and is continuing it now after the baby."

"She is very dedicated to yoga. So much that she has never stopped practicing it. She did yoga the entire time she was pregnant," adds the source. "And Christina has already been working hard on getting her bod back in shape! She is doing private yoga sessions and loves how it makes her feel."

The blond beauty "loves the way it makes her look and feel, inside and out. It's so calming and keeps her centered. She is obsessed!"

"Everything is about moderation," says a source. "She will have cheat meals occasionally too. Running after Max also is helping her burn some extra calories. There's never a quiet moment, Max is very active!"

For the most part, though, Christina is "eating clean and watching her portions," says a source. She's also breastfeeding baby Summer.

That said, Christina's main focus hasn't been shedding the pregnancy pounds. The hot mama "is all about family right now, that's what her priority is," says a source.