This could be caused by a number of things. You may have PCOS or just have a hormonal imbalance. It could also be caused by stress, illness, certain medications, etc. I would have an annual exam/pap with a gynecologist. During the exam, mention the irregular periods and any other symptoms that you have. S/he can do blood work to check your hormone levels and find out what is going on.
stress....? and why dont you think your pregnant?