I need help please someone help me

Or advice but please before I start I don’t want anybody saying mean things or judging I’m just a desperate mom. So I gave birth in December to my little boy and I have PPD ever since I don’t know why honestly but i just feel this depression consuming me slowly well it got worse ever since I found my daughter 4 year old doing something very weird in the shower. I think she was masturbating and yes I know how crazy that sounds. Why would a 4 year old masturbate? I don’t know but I caught her with her legs wide open while letting the water fall on her vagina and when I saw she got really scared and nervous like she knew she got caught doing something bad something she wasn’t supposed to and when I asked her why she did that she told me lie and lie but after trying and trying she said that’s how she loves herself. My heart broke honestly because I didn’t know what to think. She doesn’t watch anything inappropriate on tv, my husband and I always do our business in our room with the doors locked all the time so there’s no way she could have seen on us. And I’ve also caught her a few times blowing on my sons face on purpose ( babies can’t breathe when you do that). Please someone tell me my daughter isn’t the only one. Someone tell me this could be normal for her age. What am I doing wrong. Can I still fix this? Someone please help me because I’m losing my mind.