We have a black angus heifer that is about a year old.?

We have a black angus heifer that is about a year old. She is in her own pen that is about half an acre big. She is fed twice a day with a sweet feed/Corn mix. She has free feed of cane hay and she get's alfalfa everyday too. She has clean water in her pen. None of that is new to her, and there has been no change in her feeding schedule or food. There is a steer calf in the pen next to her. There are two horses in another pen around her. But none of them have access to her. Today when we woke up she is mooing very loud and running around. The mooing is non stop and she is pushing on the fences and acting strange. Any ideas what could be the issue? She ate her feed and then went right back to mooing and running around.