It doesn't happen all the time, but when it does woo boy watch out.
In the first trimester my emotional dragon liked to roar its ugly head in the form of anger. Boy did I ever get cross. Mostly at our poor dog. It was like something I never felt before, genuine, deep down anger. And at things that really weren't THAT big of a deal. Well...the poop smeared all over the carpet would put anyone in a bad mood.
Now that I'm in my second it seems the emotional trend is downward. Oh I just get so sad. Sometimes it's a commercial. Sometimes it's because my husband does something really nice and sweet for me. Sometimes, and this is probably the most common reason, it's because of a dream I had.
When you are pregnant your dreams are more vivid. I've always had pretty vivid dreams anyway so basically now every night, I live my dreams (imagining Inception here). I really think they are reality. And it won't be just one a night; it's always several. Once woken up, of course I know it was a dream, but the emotions felt in the dream still linger. Last night I had a dream my dog got hit by a truck. I spent the rest of the dream crying and depressed. Guess how I woke up feeling this morning? The other tricky thing is once you get in a certain mood, it takes a lot of cute Youtube videos of kittens to get you out of it.
So, by matter of deduction, I can only assume that having gone through phases of extreme anger, and sadness, the next one could either be dopey, happy, bashful, sleepy, sneezy, or doc. I'm rooting for happy.