Help I could cry I'm so frustrated

I NEVER let anyone watch my baby because of this reason, I let my MIL watch my son while I took a 10 minute shower! I was gone for 10 minutes in the other room, I came out after I got out of the shower & she was feeding him rice cereal that she must have brought behind my back when I've made it VERY clear he Is not eating that and not starting solids until 6 months. He is almost 5 months. I am Livid, I have never been so upset. She brought it behind my back knowing I've said no and did it while I was in the shower. He only got a couple bites but I'm seriously so upset. I made her leave and my SO said I'm overreacting 😭 opinions? Did I overreact by making her leave?