Did she know you were around and might walk in and see her, or were even in the same room?
One of my favourite things to do when I enjoyed corrupting mom's online, is pushing them to take risks around their brats. At first they usually don't even like being wet when their kids are in the room, but slowly I'd get them to enjoy being naughty when they're oblivious. Eventually I'd have them looking right into their sons and daughters eyes, repeating "I'm his slut" in their mind, and sending dirt pictures to me.
And if I was lucky, I'd push them till they did what your mommy was possibly pushed to do. I'd push them to secretly play with their cunt when their kid could walk in at any moment, or even when they're in the same room with their backs to their mom.
Maybe she did just have an itch, she is human too, but she might also be well on her way to being turned into some guy's obedient slut, putting his cock ahead of her duty to you.
If he's doing it right, next time you walk in on her playing with herself, he might order her to ignore you and keep going, and she might do as she's told. Your mom deserves to have fun too after all, and we deserve to use her however we want to get our rocks off 😂