Why do I feel like this?

I used to think I was a great mom. I was so patient and easy going with my first. I now have 3 and I honestly don’t know why I feel the way I do. I’m constantly fed up, irritable, I scream at my kids sometimes, I just feel rage. I would never hurt one, but I get so angry. Not just with them. With everything. Dropping something on the floor can fill me with rage, not being able to open a package, little things like that. Then I cry after getting so angry because I feel guilty, I don’t want to yell, and I don’t want to feel this way. I prayed for these children, I would die for them, I love them, so why do they make me feel so angry? Where is my patience? They honestly don’t deserve this.