What Causes 2 Missing Periods And Negative Pregnancy Tests?

I'm 25, 5'4, 180 lbs. I've missed two periods and had a handful of negative pregnancy test. I'm having pain/discomfort in left lower quadrant. (above hip bone) I've had three successfull pregnancies already and after the first missed period thought I was pregnant due to symptoms until all the test pointed otherwise. What could be going on? Going to the drs is a last resort option for me.