What Causes Delayed Periods And Knot In Stomach While HPT Is Negative?

hi my name is tiffany and the last time i had my period was on my weddin day, on june 19th and it lasted for the night, and i have not had once since. i took two pregnancy tests and they both came back negative, and i had a blood test that came back negative, but i have a knot in my stomach, fatigue, nausea, swollen breast with dark brown around the nipples, and i have lower back pains