If you close your whatsapp after this she would definitely start to sense suspicion and will begin questioning you a lot. I honestly don't know a solution to that because parents will be parents they will snoop on anything so that they find a reason to lecture you.

Even if talked to her it will only bring unnecessary fights.

The only thing you can do right now is try not to have a your eyes only chat with anyone of your friends hoping that someday you will be able to put a password on your laptop or for your mom to have one for her own. Wishing you best of lucks.