There are several causes for irregular periods. Examples; stress, weight gain/loss, medication, illness, climate, cyst, or polyp of the cervix, hormonal, thyroid, infection.. just to name a few.
Without a proper exam it would be hard to pin point the exact cause. Do call your doctor for an exam to determine the exact cause. If you are bleeding enough to go through a pad/tampon and hour, do call your doctor you can become anemic this way. If it's really heavy you may even want to to be seen this weekend. They can always give you something to stop the flow and then have you follow up with your doctor as to what is going on.
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