It may be that you just had not ovulated, therefore you won't get a period. However with it being 12 weeks, I would still see your doctor though. There are several causes for irregular periods. Examples; stress, weight gain/loss, medication, illness, climate, cyst, or polyp of the cervix, hormonal imbalance, thyroid, infection.. just to name a few.
Without a proper exam it would be hard to pin point the exact cause.
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