This one can be hard to process, especially if you were raised not to acknowledge or express anger. Parenting can tap into a lot of anger, even serious rage, at all kinds of people and situations. It's a combination of the parenting hormones (which parents develop from contact with their kids, so parents who didn't give birth to their kids also develop these hormones) and the intensity of the love and stress of caring for kids. Suddenly you see things with new eyes, and you're more attuned to vulnerability and injustice, to power imbalances and to cruelty of all sorts. All that can tap into anger you never knew you had (or had already worked to balance or overcome).
It can be scary, this anger. You may not feel anger at your actual child until they're older than baby age (or maybe you do--that's ok, too), but the feeling of rage at the injustices in the world and how little control you have over things can wash over you when you least expect it and feel least equipped to deal with it. It comes in waves, and tends to crest whenever your child is entering a new phase that requires new skills or focus from you. And then once you get your feet under you again it'll subside.
It's all normal and ok, unless the anger feels like it's taking over your emotions completely and preventing you from engaging or enjoying things. If the anger feels like it's bigger than it should be, talk to someone. This is a symptom of hormonal imbalance or PPD (men and mothers who didn't give birth also get PPD), and you need to talk your way through it with someone who knows how to help you deal with it and get it back to its proper, manageable mass in your life.
(Don't forget Day 1: You can do it. You can do it and you are doing it.)