15 month old out of control

Hi everyone

My 15 month old came out a complete different baby from my 9 yr old. 9 yr old never cried was very chill. 15 month old is just mean and screams and cries all day. I honestly don't know what to do with her. She is mean to the point where my 9 yr old is scared of her because she always tries to attack her for no reason. I mostly stay at home with my 15 month old. I thought maybe she was bored so I would go run errands with her and all she does is scream in the store to the top of her lungs kicking and screaming for no reason. Always check to see if she is hungry,wet or tired but it doesn't matter what type of day she is always screaming. I say no no to her at least 100 times a day. Somebody help we're not even in the terrible twos yet. This started around 11 months old