Oh dear HoneyLicious, please take care of yourself. I was pretty emo a few weeks back (probably becoz in my first trimester so feeling very uncertain)
Changes in hormones, family history of depression, stress at work or family life, lack of rest or support, all these triggers depression. Usually the gynae will prescribe medicine to help you relief the depression or recommend a therapist.
I read some books about during and after pregnancy depression. Too much anxiety will lead to depression, and this will increase risk of getting depress after giving birth as well.
Depression can harm the baby so for your baby and yourself, cheer up and I hope you overcome the unhappy feeling soon! ^_^