The singers started their careers together as children in The Mickey Mouse Club, along with Spears' ex Justin Timberlake and Ryan Gosling, and rose to fame in their music careers alongside each other in the 90s.
Although Aguilera has spoken out in defence of Spears before, in an Instagram post Spears blasted her old co-star for not answering an interview question about the recent end of her conservatorship.
The 39-year-old singer shared a clip of Aguilera being pulled away from a red carpet interview by her publicist who said "we're not doing that today" after she was asked about it, with Aguilera then telling reporters: "I'm sorry, I can't, but I'm happy for her."
Spears wrote on her Instagram Stories alongside the clip: "I love and adore everyone who appreciated me... but refusing to speak when you know the truth, is equivalent to a lie!!!
"13 years being in a corrupt abusive system yet why is it such a hard topic for people to talk about?? I'm the one who went through it!!!!
"All the supporters who spoke up and supported me thank you!...Yes I do matter!!!"
However, Aguilera, 40, had supported Spears earlier this year before her conservatorship hearings.
She wrote on Instagram: "To be silenced, ignored, bullied or denied support by those ‘close' to you is the most depleting, devastating and demeaning thing imaginable."
Aguilera added: "Every woman must have the right to her own body, her own reproductive system, her own privacy, her own space, her own healing and her own happiness."
In the story following her rant about Aguilera on Instagram, Spears shared a clip of Lady Gaga telling reporters: "The way that she was treated in this business was really wrong and the way that women are treated in the music industry is something that I wish would change.
"I think that she will forever be an inspiration to women."
Spears wrote: "Thank you Lady Gaga for genuinely taking your time to say something so kind. You made me cry!!! I love you!!!"
The singer had been living under a conservatorship until it ended last week, after it was put in place in 2008 following a breakdown.
Recent documentaries had chronicled the #FreeBritney campaign to end the conservatorship which saw her father James Spears controlling much of her life.
Watch: Christina Aguilera avoids Britney question at the Latin Grammys