Stay at home dad and depressed...

Me and my wife had our 3rd baby 3 months ago. She went back to work at 8 weeks by choice. She's a gynecologist. I've been a stay at home dad since our first baby. Before that I was a teachers aid. This has nothing to do with my kids, but I have been feeling this overwhelming... Depression and Idk why. I was fine with having a 3rd kid and she was planned but I feel so guilty admitting this but when she was born I didn't feel... Happy... My wife was crying tears of happiness and all of a sudden the second I see her thus weird depression washes over me and I feel like a terrible father for thinking this. I thought maybe I'm nervous, but it hasn't gone away. I don't have any ill feelings towards my daughter and I don't know the root of the depression. Idk exactly what's making me depressed. I've just gotten washed over depression when she was born and it never went away.