There are several reasons a woman can bleed when not on her period, including implantation bleeding and hormonal irregularities. Sometimes, it's normal to bleed outside of a normal time, but other times it can be a sign of a health issue. Some women spot during their cycle as a normal part of their menstruation.
According to Women's Health, normal menstruation happens around every 28 days, but it's possible for it to happen between 21 and 35 days apart. That's considered to be normal, but if a woman is under 11 or post-menopausal she may want to speak with a doctor. There are other reasons, like a miscarriage or low thyroid levels, that could result in abnormal bleeding.
IUDs also can cause occasional bleeding or spotting, so if one is implanted, this could be the case. Malignant cancers and injuries to the vagina could also cause bleeding.
A number of GYN procedures can cause bleeding as a result of taking biopsies or completing a D&C. Some bleeding is normal in these cases, but any heavy bleeding should be reported to a doctor.
Hormonal fluctuations can cause bleeding at irregular intervals, and implantation bleeding or pregnancy may result in spotting when the fertile egg implants. Missing oral contraceptives or estrogens can also cause hormonal fluctuations that could be the cause of odd or irregular bleeding.