What should I do if one dog starts bullying the other?

I have a male 1 year old chihuahua named Beast. I also have a female Dashound, soon to be a year old. I have always fed them the same food, at the same time everyday, and right next to each other. However Within the last month I switched from puppy food to adult food, and Beast has been aggressive to my Dashound Leia, 3 days in a row. More on What should I do if one dog starts bullying the other?