Feeling very down and sad

I'm 22 weeks pregnant, I work full time, I'm a part time student too, and lately I've just been feeling so down and emotional. Like I can literally go from having a really great day to feeling so down and depressed and emotional. Is this normal in pregnancy? Or have I just got too much going on and it's my body's way of telling me I can't cope no more. I'm afraid that these feelings won't pass when I have my baby too. Please help x