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Any sudden changes in the regular menstrual cycle can be due to stress,exertion,hormonal fluctuations,drug intake or even due to overwork.If the periods are unusual every month then it can be indicative of any underlying uterine or ovarian disorder.
Since you are planning for a baby consider visiting your Healthcare Provider and get a complete pelvic examination done along with USG scans and hormonal analysis so that you can plan accordingly.
Take Care!