What Causes Missed Periods, Abdominal Pain And Swelling Of Uterus?

It's been 3 months since my last menstrual cycle. I took a pregnancy test on the 2nd month that I missed my period and it came out negative. I never had any pain till these past 3 days, I feel a light pinch on the left side next to my belly button. And I feel like if my uterus is swollen. No weird discharge except for the clear mocus. whats happening to me?