Thanks for the query.
As you are having unprotected intercourse
the possibility of pregnancy
But just by symptoms or by altered menstrual flow we cannot confirm or exclude the pregnancy.
So you once go for urine pregnancy test to find out the possibility of pregnancy.
If the test comes negative the cause of menstrual irregularity could be something other than pregnancy like hormonal
Better to use some regular contraceptive
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429