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Irregular periods could be due to various causes as Ovarian cysts,hormonal imbalance,Fibroids,Pelvic disease,Thyroid disease etc.Since you had unprotected intercourse chances of pregnancy are there even if you have menstrual irregularity.
Also urine pregnancy tests are not reliable as they give false results.You require a proper pelvic examination along with hormonal evaluation and USG scan for ascertaining the cause of irregular periods.Even if you are not pregnant kindly visit a Gynaecologist for evaluation.
Take Care!