German blue ram suddenly aggressive toward fish?

I have 12 cardinal tetras, 1 german blue ram, and 1 betta in my tank. I had the cardinal tetras first, then I got the ram, then the betta last. The first few days and weeks, my ram and cardinal tetras would never ever bother each other. They would sometimes even be less than an inch away from each other but there was no aggression. I started noticing last week that my ram would chase the betta sometimes. And today, about 10 minutes after I fed them, one of my cardinal tetras fins were torn. I'm almost positive it was my ram. About 1/3 of its fins are torn off. Why is my ram acting so mean all of a sudden? The cardinal tetras do hog food a lot, but she does get her share. Plus, she gets all the food that sunk to the floor and eats it because the cardinal tetras never touch it once it hits the ground. She's always been scavenging the floor, even the when I literally first put her in my tank. The first thing she did was go on to the floor and started munching on planaria.