There is such a thing as ultradian cycling, which can cause people's moods to swing rapidly and even multiple times in a day. I think it's fairly rare though. More likely, you may have some metabolic process going on (such as a physical illness or an electrolyte imbalance), severe stress or anxiety, or you're not taking good care of yourself. All of these can make your moods jump up and down like a frog in a hot skillet. But you need to talk to your doctor about these things as he is best qualified to decide what to do.