Plan B pill induced menstrual cycles

Referring to the data you provided (sexual intercourse one week after menstrual period was over, took plan B pills, headaches, being tired, cramping, and bleeding that resembles menstrual bleeding but much lighter that started one week after the pills), you might be experiencing a menstrual period caused by the Plan B pills. The birth control pills contain hormones similar to the ones in the woman's body and interfere with them. Therefore, when using birth control pills (when starting, stopping, or taking them on occasion-like in your case) changes in the menstrual period and/or the menstrual cycle might be expected.

The changes might include: change in length, change in blood loss, and change in blood (lighter or heavier). This is a temporary condition and is not supposed to repeat during the next menstrual period. If it does, you might want to visit a gynecologist for a physical examination and hormonal laboratory tests which would tell the doctor what hormone-containing pills would be best in order to restore the normal hormone production pattern. The gynecologist would also check if you are pregnant before giving you hormone- containing pills which are not allowed to be taken in pregnancy