Hormones. They fluctuate and change at night. What you're feeling is perfectly normal, and it will pass. I felt it with both my babies. With DD1 I cried and cried because I felt I'd ruined our lives and my relationship would never be the same (and it's not, but that's not a bad thing!) and then with DD2 the same...how could I handle this, what have I done etc. It will pass. DD2 is 10 months old now, and I'm so overjoyed that both my girls have each other. It's a beautiful thing to see your children love each other.