Because having a baby is a HUGE adjustment. Your time is not your own. 24 hours a day goes into this little one. Even if hubby helps there and there, you've gone from 24 hours being your time, to what, 20 mins a day being your time? Plus you've got hormones. Think about what you need. When you take a break, use your time wisely. Don't do housework in your break time. Ask for MORE help.
You’re doing an amazing job! It’s totally normal. I feel the same way, I thought I was the only one.
Postpartum is hard. It effects everyone differently. Sounds to me like you’re a great mom and doing a wonderful job. Take it easy on yourself.