I have never heard of a 4-5 month old masturbate but the way you described her behaviors, that is what it sounds like to me also. Call me paranoid, but I would be careful about who I share that video with.
Are there seizures that would put you in this same physical position over an over? Who knows? Good Luck, and I hope you share what you find out.
She sounds to me like she's masturbating.
Is that possible?