Selena Gomez isn't taking this online bullying lying down -- she's sending a HARSH message to Hailey Baldwin after Mrs. Bieber posted a screen shot of the single "I'll Kill You" following the release of Gomez's single "Lose You To Love Me" yesterday -- which is clearly about her painful breakup from Justin. Ouch!

We spotted Selena out at private club The Bungalows last night, celebrating the release of TWO new singles and just after posting a reply to Hailey ...

Fans went crazy after an eagle-eyed Selenator noticed Hailey's post just four minutes after Selena released her new music video yesterday and of course everyone assumed it was a "response" to Gomez's song. Hailey addressed the accusations by writing, "Please stop with this nonsense...there is no 'response.'"

But that wasn't enough for Selena fans or for Selena, who responded herself late last night by saying, "I am grateful for the response that the song is getting. I’m so grateful. However, I do not stand for women tearing women down. And I will never, ever be by that. So please be kind to everyone. It doesn’t matter what the situation is. If you’re my fans, don’t be rude to anybody, please." She went on, "Don’t go off and say things that you just feel in the moment. Just please, for me, know that that’s not my heart. My heart is only to release things that I feel are me, and that I’m proud of. And that’s all I’ll say." Daaaaang!

Then, moments later, she dropped ANOTHER new single called "Look At Her Now." Check it out!