Hi Welcome to HEALTHCARE MAGIC..
I have gone through your query and can understand your concern...
Yes
stress could cause delayed periods. Stress could alter normal
hormonal balance & could cause delayed periods. Delayed periods could also be as a result of hormonal disorder, some ovary problems,
excessive weight gain, over-exercising in previous month & many other reasons & not
pregnancy.
As you had sex on 31st January & had your
menstruation after ward in afternoon which continued till 4th of February, you should not get pregnant by the intercourse you had on 31st of January.
But if you had unprotected sex after 31st of January, then you could get pregnant & then delayed periods now could be because of pregnancy. If you haven't had any sexual intercourse after 31st then you should relax you are definitely not pregnant.
Refer to a gynaecologist for more queries
Don't worry you would be fine
Hope this information helps