I don’t even know where to start

My 4 year old has been severely acting up lately. We took her out today, where she acted horribly. She fell asleep in the car for a little and when we got her into the house to lay her down, she was exhausted..I mean beyond exhausted. I tried to get her to finish nap. She started screaming and screeching. I tried to calm her down and talk to her but it didn’t last long and she returned to the screaming and screeching, flailing throwing stuff. I remained cool and calm for a few minutes, but the screeching got under my skin, I told her to stop, that she is 4 years old and doesn’t need to behave like his over a nap. When she didn’t stop, I reached over and slapped her in the face. She was so shocked and curled up and started screaming more. I immediately grabbed her and apologized and sat there and rocked her until we both calmed down. I have never felt like a worse parent and have beaten myself up all night. She has been giving me love and wanting to cuddle and everything seems normal. But I look at her and wonder how I could lose it like that!