Ok guys, it's not that rare...
It isn't a "real period" but is in fact a period like bleed.
It is usually from either:
a) endometrial tissue in the cervix or around the surrounding area.
B) cervical abrasion - rough sex, cancer or other things
c) placental abruption - needs medical investigation.
There are tother causes but these 3 are the most common.
Any bleeding during pregnancy should be checked out by a health care professional