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If you are late by 2 weeks and the pregnancy tests are negative then you are a case of oligomenorrhoea .The causes range from hormonal ,psychological,stress conditions to mechanical. One should maintain a menstrual calendar for atleast 3 months to study the pattern .A single episode can be more easily attributed to stress in light of previous regular periods.
However,it is likely that the discomfort and symptoms you are facing are part of premenstrual syndrome and your menses should be starting in few days.
I hope i have answered your query.
wishing you health.
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