In fact, the 37-year-old actress confessed that she was "salty" about Shia's rise in Hollywood after they played siblings Louis and Ren on the Disney Channel show Even Stevens from 2000 to 2003.
In a new video on her YouTube channel titled "Why I Don't Talk To Shia LaBeouf," Christy said the pair have a "sibling rivalry" and "weren't really close" on set.
"We had a little bit of animosity," she shared. "I always wanted him to really appreciate what I gave to him."
One incident that she said "really hurt my feelings" was when the Honey Boy star, now 35, snubbed her after he won the Daytime Emmy Award for Outstanding Performer in a Children's Series almost 20 years ago.
She recalled, "He was on the podium. I was sitting there with the rest of our team, and he thanked like everybody at the table. But he didn't thank me."
Obviously, Christy said, "I was hurt at the time, because I felt like since day one, it was him and me. It was, like, our show. But because it was so life or death or him, it was his show and I was just like around because I was a girl."
After the TV series ended, they went from seeing each other every day to not seeing each other for years. He became "a huge star," as Christy put it, with roles in Transformers, The Peanut Butter Falcon and Pieces of a Woman.
"For a while, I think the ghost of who Shia was and who he became was really frustrating for me. I didn't watch any of his movies. I couldn't. I couldn't bring myself to watch them," the Kim Possible alum admitted.
"I was a bit salty," she revealed, saying she felt like he had gotten lucky by landing a better agent and better manager than she did.
"So, I kind of felt a little jilted by the whole situation," she went on. "Here he is, making a big old splash in Hollywood. And here I am. I chose to go to college. And there's consequences that come [with] that but, like, there was definitely an undercurrent of regret, but also an undercurrent of comparison. Sibling rivalry, if you will."
However, she said she doesn't blame Shia as much anymore, saying her feelings have "really mellowed out" over time. "We just went [down] completely different pathways of life."
There was at least one silver lining of Christy's decision to study film at Barnard University. While taking classes, she met her now-husband Brendan Rooney and they later welcomed daughters Isabella, 4, and Sophia, 2.
But all these years later, Christy still feels that she and Shia have a long-lasting connection as former child stars.
"I guarantee if I saw him walking on the street, there would be a bond there that is undeniable," she explained. "When you grow up with somebody in that particular way... you're bonded together. You can't help but want to see the person succeed."
Watch her full video above.