Thanks for the query.
There is possibility of pregnancy
as you had unprotected intercourse
But just by symptoms and scanty bleeding we cannot confirm or rule out the pregnancy.
So you once consult gynecologist and go for ultrasound
It can find out the possibility of pregnancy and in case of absence can find out the cause for your irregular periods
Meanwhile take symptomatic treatment.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429