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Increased stress, diet changes, changes in sleep pattern, can cause hormonal changes causing delay in periods. Variation of 5-10 days in menstrual cycle length is considered to be normal. If you had been sexually active during fertile days, chances of pregnancy are a possibility (condoms are associated with failures). Wait and watch for your periods. Visit your doctor/gynecologist for further evaluation if periods do not start within 10 days or if you suspect chances of pregnancy, or if you develop any other new symptoms. If pregnancy gets ruled out, and if you are not ready for pregnancy, but intend to remain sexually active, you should consider visiting your gynecologist to get started on suitable regular birth control measures to be protected (after your periods). Be regular with birth control use as instructed. Avoid having unprotected sexual contact if you are not ready for pregnancy.
Take care.