"I'm just a girl" is an ironic catchphrase used as a meme in TikTok videos and elsewhere as a response to "boys will be boys." Both phrases excuse a type of undesirable behavior, but "I'm just a girl" repurposes a sexist view of women as incompetent, naive, and without full adult-level agency while the other excuses all bad behavior.
The phrase is often accompanied on TikTok by the 1995 single by rock band No Doubt titled "Just a Girl" which lashes out at a culture that treats women as fragile and incapable.