Thanks for the query.
As you are on birth control the possibility of pregnancy
is less if you used the method according to the prescribed schedule.
The symptoms you are having could be to some infection as you said.
Disturbance in general health can affect the menstrual cycle.
So after consulting your doctor go for blood test
as you planned.
If the test comes negative, go for one ultrasound
If everything is normal you can go for withdrawal bleeding.
Meanwhile take symptomatic treatment.
For more details you can ask me through: http://www.healthcaremagic.com/doctors/dr-sree-gouri-sr/63429