Sudden, extreme depression

I'm 18 weeks and up until like a week ago I've been completely fine my whole pregnancy. Suddenly, I'm extremely depressed and get anxiety attacks over made up scenarios in my head. It almost seems like the extreme depression started when my breastsvstarted leaking. I just feel so down all the time. I always feel on the verge of tears and I always have this pit feeling in my chest.

I'm going to seek therapy, but I just don't understand why it came on so suddenly. I'm in an amazing and healthy relationship with a supportive and loving partner. I have a wonderful 7yo and this pregnancy has been such smooth sailing!