Adult Cat Stopped Eating?

My approximately 1 and a half year old adult cat used to eat A LOT. We used to feed her nearly 4 servings a day and in between every meal or so, she'll eat her little chicken meaty cereals. About a month ago, we took in another baby kitten. They're getting along etc. They play with each other every day and if one is missing, the other will whine until the other appears. So maybe it's not the stress part for my adult cat, but all of a sudden, she doesn't eat her big meals anymore. she'll eat a few cereal pieces and continue with her day. we have separate bowls for both cats even though the little one will occasionally dig into the older one's bowl for some more, but what truly is the problem with why my cat won't eat? before all this, she used to be a bit on the heavy side, but at this moment, she weighs nearly the same as when we found her which was fairly light. should i feed her different food? is she secretly stressing about the new kitten? please help me.