Since you had irregular periods
before this please understand that it can be because of hormonal
imbalance.As you did pregnancy
test twice & is negative so there is no chance of pregnancy.Now as for the pain in abdomen it could be due to few causes like:-
-pelvic inflammatory disease
- could be a intestinal pain
(Poly cystic ovaries)
the most common occurring in sexually active is probably pelvic inflammatory disease also called PID.Please see whether there is any vaginal discharge;also you can take an anti-biotics & see.Better if the pain is not reduced please consult a doctor even before starting antibiotic.