Try a different prenatal. I had that problem with my first pregnancy.
I'm 17w 1d, my first few wks I cried constantly. Sometimes even at work I would tear I'll and have really sad thoughts, I got to a point where I finally had a little break down in front of my sister and got it all out, I felt much better. Every time I feel like I might have a low point after that, I would call my sister and almost immediately feel better. Pick a support person and talk to them about it, don't go through the emotions alone...you'll just be sad all the time. Good luck :-)