I don't know if I should be mad

I don't know if I should be mad at my Mom. I came in to my room to hear my son screaming. Up from his 5 minute nap. He was flailing his arms and crying into the sheets. My Mom just put him down before I came in. He was on his belly kind of diagonal in the bassinet.

She always says my sister and I slept better on our bellies. Before the 90s it was the norm and she still feels it's best.

I wouldn't put it past her to place him belly down. She tells me he probably rolled and woke himself up. She swears he was on his back when she left. He hasn't rolled yet so I am very sceptical.