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All birth control measures have failures at time (incidence rate varies in each method). The delay in periods and light bleeding could be suggestive of pregnancy. Other possibility of bleeding after intercourse could also suggest vaginal and cervical infection, chronic cervicitis, cervical polyps etc. Periods can also be delayed due to stress, change in lifestyle, exercise level changes etc. The exact cause for your symptoms can only be made out by a careful examination, blood HCG test and hormonal analysis, vaginal and PAP smear test. Visit your gynecologist at the earliest for examination and further investigations to confirm the cause for your symptoms. If pregnancy is ruled out, appropriate treatment would be started based on the investigation results to help you to regularize and get back periods on track.
Take care.