First let's understand that
Implantation Bleeding has not been scientifically proven.
If you have been bleeding for 15 days, you should be making a doctor's appointment to see what is happening. Bleeding for 15 days does not mean that you are having twins, but could be quite the opposite actually.
Please make a doctor's appointment to find out what is going on. Whatever info you do get here could be harmful to your health as we do not know your history and have not done a physical. So all of your answers will be guesses at best.