4 year old daughter is humphing !!

So my husband and I caught my daughter humphing her stuffed animals. She tends to hide under her covers and place them in between her legs and begins to humph. I’ve spoken to her on several occasions and explained to her on why she can’t do that. I’ve sat her down and asked if she’s ever seen anyone doing it or if anyone has done it to her but she denies it. When I do catch her and confront her she either says it was itchy or that it helps her be able to pee. I don’t want to reprimand her, or yell and make it a big deal. But I’m honest at a loss me just speaking to her isn’t working cause it keeps happening. Has any other moms gone through this ? how did you handle it ? did it stop ? is 4 too young to be doing this ? Her tablet is connected to my phone where everything she watches on YouTube I have access too so I know it’s not from her tablet. Google states that’s its normal but is it really ? !! Any advice will be greatly appreciated.