Pregnant symptoms but negative result

I have implanon birth control for 3 years next July. My last period was August 8-12 I started having cramps and being super emotional and moody the end of August thought it was my period getting ready to start plus I've had bad cramping. I still have not got my period yet. I took a pregnancy test "clear blue" from DG about 3 weeks ago. I've had very bad cramps and some lower back pain and been super exhausted lately. I work 3rd shift thought it could be that. I've had a daughter she just turned 3 I've had so many symptoms like before but the negative test threw me off. Could it be something going on with my birth control?? I called the DR they told me to take another test in a couple days not from dollar general....