Period A Month Late With Pregnancy, What Would Be Causing These Symptoms ?

My period is not one to be called regular. Every since i got it its been normal to be 1-2 weeks late. However it has never been this long, im now a month late. I am sexually active. Hes never gone inside me before and im not on birth control . I took a two pregnancy tests (first response) that came back negative, and saw my Dr. who told me I was just experiencing irregularities. For almost the entire month now, I have been experiencing bloating , fatigue, stomach cramping, breast pain and swelling only in one breast and mild nausea . I m wondering if not pregnancy, what would be causing these symptoms ?