Spotting is irregular uterine bleeding that has to do with the use of contraceptive hormones.
If you loose just a few drops of blood it’s called spotting, with a lot of blood it’s called a breakthrough bleeding.
This is quite common if you start using birth control pills. It occurs in up to 30-40% of the users. The first months of use the lining of the womb becomes thinner and your body is adjusting to this. It can be inconvenient, but it’s nothing to worry about: the pill is still reliable.
It can also occur if you use oral contraceptives and try to postpone your period by skipping the pill free period. It may also result from inconsistent pill talking.

If you started using oral contraceptives and you still have irregular blood loss after three months you might ask your doctor for a different brand, changing the amount of estrogen and/or progestin. If you had unsafe sex and bleed irregularly it could be causes by a STD. You should have this checked out.

See also: uterine bleeding