How can I tell the difference between my moods being just my hormones or something more serious?

Watch the timing. If your mood gets "bad" post ovulation, about 2 weeks into your cycle, or later and gets better as soon as your period starts. It may be hormonal. Keeping a healthy diet and exercising can help alot. If you are a candidate, birth control pills often help with pms. Even if it is "just pms" it can be debilitating. You should see your doctor for treatment.
If you are thinking about hurting yourself or others, please seek psychiatric help immediately.