What Causes Missed Periods, Nausea And Tender Breasts?

ME AND MY FIANCE HAVE BEEN TRYING TO HAVE A BABY FOR ABOUT A YEAR NOW I HAVENT HAD MY PERIOD IN 3 MONTHS I HAVE HAD NO SPOTTING OR ANYTHING BUT I AM EXPIERENCING MANY OF THE SYMPTOMS OF PREGNANCY LIKE LIKE NAUSEA (LITTLE VOMITING) HEADACHES,TENDERNESS IN THE BREAST, THE AREA AROUND MY NIPPLES GOT DARKER, WEIRD STOMACH PAINS, AND MORE BUT I HAVE TAKEN 8 HOME PREGNANCY TEST AND WENT TO THE DOCTOR TWICE LAST THE LAST TIME I WENT WAS ABOUT 1 MONTH AGO AND ALL THE TEST WERE NEGATIVE. IS THERE STILL A POSSIBILTY I AM PREGNANT? IS THIS COMMON? SHOULD I BE WORRIED AT ALL ABOUT THIS AND WHAT SHOULD I DO