Hi there ! From your history and symptoms the following may be considered:
1) Birth control pills are not a hundred percent guarantee against pregnancy
. It could be that you might have conceived which was not detected and there was a miscarriage
with residual parts of the fetus left in the uterus
. This per se can give rise to spotting or abnormal bleeding, and whenever the uterus contracts to empty, this may cause pain in the lower abdomen.
2) Your present symptoms and signs however points to some kind of peritonism, which can occur due to pelvic infection
due to retained products of conception which acts a foreign body giving rise to infection. In that case you should get yourself examined by a gynaecologist who may advise you to get your serum HCG and an ultrasound
abdomen done which will help in either diagnosing or excluding these aspects and accordingly appropriate management can be inititated.
3) WIthdrawal bleeding once off from the oral contraceptive pills
could be a finding, but in this case looks less likely.
, UTI, etc are other possible diagnoses which should be kept in mind, but in the present circumstances are less likely excepting for a coincidental co existing pathology.
Hope these points will help you in solving your problem. WIsh you an early reconvery.