Kevin Federline has been firing shot after shot at Britney Spears as more info comes out from his upcoming tell-all … but now Britney is fighting back on X.

The Grammy winner wrote, “I 100 percent beg to differ the way he is literally attacking me in his interviews.”

Read the whole thing here … according to her, it’s obvious why he would want to hurl false rumors about her. She claims he hasn’t gotten any money from her in 5 years and that he’s “trying to get paid.”

In his forthcoming memoir set to publish next week, “You Thought You Knew” … K-Fed alleges Britney cheated on him with a female dancer and did drugs while still in the breastfeeding phase with their children, among other things. Kevin writes that her actions had a negative effect on him and that he loved her, but Brit says this isn’t how you show it.

“If you really love someone then you don’t help them by humiliating them,” she wrote.

Brit went on to hurl some accusations of her own, saying, “What scared me was how serious and angry he got, people have no idea, it is way worse than anyone could imagine… the boy hates me and it is deep anger to literally say the things he is saying.”

Britney’s reps gave us a statement about the book earlier this week … “With news from Kevin’s book breaking, once again he and others are profiting off her and sadly it comes after child support has ended with Kevin. All she cares about are her kids, Sean Preston and Jayden James, and their well-being during this sensationalism. She detailed her journey in her memoir.”