Every pregnancy is different, but you're right that (unfortunately) being emotional is often a big part of pregnancy. Your body is going through some pretty crazy changes, and it can be especially trying during the first trimester when you're just getting used to the transformation.
While mood swings during pregnancy can be traced to everything from fatigue, physical stress and metabolic changes, the main culprit is your hormones. During the first few months after conception, the levels of hormones like estrogen and progesterone change dramatically, which has significant effect on brain chemistry. So if you find yourself spontaneously bursting into tears or breaking into a laughing fit, don't worry — you're pregnant, not crazy.
For most women, the moodiness is most noticeable in the first few months of pregnancy and the last weeks leading up to delivery. If yours seem extreme or are affecting your way of life, talk with your doc. She'll be able to guide you towards further care.