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Based on the information given, you seem to have cycles lasting 31 days. Period length is different from person to person and it can be also be affected by many factors that are external such as stress, weight changes, and some medical conditions. It is recommended to see a gynecologist if you have any problems with your mens cycles

it could be a sign of PMS or implantation. Nevertheless, pregnancy is not proven without a pregnancy test or seeing a gynecologist.
 

It is essential to note that unprotected sex could cause unwanted pregnancy and there arise the possibility of getting sexually transmitted infections. The change in menstrual cycle might be a result of hormonal imbalance, which in this case should be evaluated and treated by a gynecologist. A gynaecologist should be consulted for a thorough examination of the issue and a range of suitable contraceptive methods should be discussed.
 

Worrying when pe­riods are late after unprote­cted sex is normal. Stress can disrupt me­nstrual cycles, yes. But pregnancy or hormonal shifts might also cause­ delays. Anxiousness or tension indicate­ stress. Taking an at-home pregnancy te­st provides certainty. Managing stress is ke­y too - exercise, confiding in frie­nds. Periods will return when the­ root issue resolves.