Some girls do continue to get their period when they are pregnant. If it is heavy bleeding or abnormal they may need to be concerned. Still consult your physician to let him or her know that you are getting your period.
It happens quite often do to stress levels. Some fetus' can grow what seems to be fast for your body and your body is just trying to clean itself out and make growth less hurtful to the body. Some women take birthcontrol for long periods of time tend to have periods because the body remembers that it's supposed to. There is no indefinite answer as to why. From the information I have gathered from past experiences a woman who has their period while pregnant most always has a baby girl.