According to the data you provided (irregular menstrual cycles and menstrual periods, used plan B, late menstrual period), your girlfriend might be experiencing menstrual period irregularity due to a hormonal imbalance.
Plan B is an emergency contraception pill containing levonorgestrel. It is a hormone that prevents ovulation and fertilization. It might additionally prevent implantation. Once the implantation is started, the pill is ineffective. To obtain optimal efficacy, the first tablet should be taken as soon as possible within 72 hours of intercourse. The second tablet must be taken 12 hours later.
The hormone from the pill might have additionally interfered with the regular female hormone production, which has caused the irregular menstrual bleeding.
Your girflriend might want to visit her gynecologist for a physical examination and laboratory hormonal tests, which might detect the hormonal imbalance. Medications containing hormones might be recommended to try to restore the regular female hormone production.
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