Is pregnancy possible on the implanon?

i haven't been on the implanon very long i just had my son in dec. of last year got on the implanon in january. had my period in february.but spotting, missed my period in march and now april also. I have gained some weight feel very emotional and needy, yet sometimes i want to be alone. my hips keep popping like they did with my first pregnancy and i have severe back pain.This morning i woke with bad heart burn and dying of thirst constantly. I took a pregnancy test a month ago that said negative. but what worries me is i had sex with my husband before i got the implanon put in. now what concerns me is if i am pregnant and the test wont show i am then how will i truly know if i am. They wont give me and ultrasound with negative results with the pregnancy test.I breast feed my son and wonder if that may decrease some pregnancy symptoms. What do i do? And can i be pregnant?