Generally, getting your period is a slam dunk sign that you're not pregnant. However, vaginal bleeding can accompany (or be a harbinger of) a miscarriage. Or (because I don't know your age, I'm throwing this out there), it could be a sign of perimenopause, when periods can become erratic.
Call your doctor and ask if anything I've written doesn't seem to apply. Also consider the last time you had sex. If the last time was before your period, and your periods aren't too regular and/or you're of an age where perimenopause is a very real possibility (40s, but there are women who start in their mid-30s. It varies), then that may be what it is.
Of course if you get bad abdominal pain, call your doctor.