Why so sad?

I don't know why the last few days all I want to do is cry! I don't want to go anywhere or do anything! My hubby is going crazy with worry & I've tried to not cry around him, but that's very hard. I'm 25 weeks & this is not my 1st pregnancy, but the only 1 that has made it this far! Up til a few days ago, I was nothing but happy & excited!! I don't know what's wrong...