What Causes Missed Periods, Swollen Lymph Nodes, Sore Breasts And Constipation?

Hello. I'm 27 years old, have always had a regular period, NOT overweight, & in good shape. I have now missed two periods, lymph nodes are swollen in groin area, tired a lot, firm & growing lower abdomen, sore boobs at times, pain in legs at times, constipated, & many other things. I noticed in June that my body was acting totally different. All hpts are NEGATIVE!! Confused & scared. Any advice or opinion?