I don't know what's wrong with me

My baby's 5 days old and he's been great. He eats well, sleeps well and isn't a screamer.

I'm fine during the day. But when it starts getting dark, I get into a strange mood. I'll be standing there doing the dishes after dinner and just start crying, full blown sobbing and I don't want my SO to know so I try to suck it up and stop but then throughout the night I find myself starting to cry. I don't know what's wrong. I feel like an idiot. 😞