Thanks for the query.
Menstrual cycle can undergo fluctuations depending on many factors.
So your scanty flow and pain abdomen can be because of causes like hormonal fluctuations, pelvic inflammatory disease
But though pregnancy test report was negative, if you had unprotected intercourse
around the time of ovulation there is possibility for pregnancy.
Because urine pregnancy test kits sometimes can give false negative reports.
So you better undergo blood test
for pregnancy, if comes positive ultrasound
is needed to rule out the possibility of ectopic pregnancy which can present with these symptoms.