You don't mention your age range...if you are anywhere between 30 and 50 you may be starting to go through perimenopause. If you are younger and sexually active, it might be possible that you had a miscarriage...or it might not be either of these possibilities.
Women's health is very complex and irregular periods can be a sign of too many different things to address in this forum. It is always a good idea to see your gynecologist anytime you notice something out of the ordinary with your periods.
Depends on your age. If you have not been having periods for too long, it is not uncommon. If you are in the age bracket of perimenopause or menopause, it is not uncommon. If you are sexually active and could possibly have been pregnant, you may have miscarried. You need to call your doctor first thing in the morning to go the Emergency Room tonight. Depends how heavy you are bleeding.