Thanks for writing in.
The most common cause in your case is side effects of I pill
It is caused by high dose of hormones in the pill.
After your normal cycle duration you will have periods that is from next 17th.(17 oct)
As you have bleeding after 4 days of taking pill chances of you getting pregnant are almost zero.
In future you should practice other measures of contraception
.Using the I pill everytime can have serious disturbunces of menstrual cycle.
I encourage you to visit a qualified gyanecologist for proper advise and treatment.
Hope this helps.