Hi thanks for your question.
Though withdrawl method doesn't give 100% protection against pregnancy
but still chances of pregnancy are less because you got your periods on due date,though pattern of periods was little altered.Since the method of contraception
you have used is not 100% secur and your periods were light,the possibility of this bleeding as implantation
( bleeding that occur at time of implantation of fertalized ovum into uters) bleeding cannot be ruled out.I would suggest you to get you home pregnancy test done 10 days after this bleeding to rule out or confirm pregnancy.Itching in vagina nausea
and being sick could be due to pregnancy or may have different etiology and needs consultation from a gynaecologist
Hope this answers your question.