First off, take stock of your life! Are you going through any new or unusual stressers right now? Moving? Changes in school? Changes in social status (student to working or some such thing)? Maybe a friend or family member is going through some major event - like an illness - that affects you in some way ? It doesn't have to be a quick change. Even something that seems long and drawn out or several months away may be stressing you out now.
And any life event, no matter how much of a handle you may think you have on it, might be putting you under more stress than you realize and causing your emotions to go more than a little haywire. So keep that in mind before you even consider letting a doctor put you on some weird medication that may help or may just make things worse! You don't want to be on a long-term symptoms-management drug for what may be a short-term problem.
There are a ton of things that could cause something like this. Probably the most common and least worrisome would be a hormonal imbalance.
You could go seek professional help if you feel it is urgent. Or if it is not putting you into a giant panic just wait it out a little bit and see if anything changes on it's own. Could be one of life's many unexplainable mysteries that simply needs time to improve. Just use your best judgement.