What Causes Absence Of Periods While Having An Implant In Arm?

I have an implant in my arm since April this year, have had no periods, weight gain, feeling of movement in my stomach with a dull ache similar to that of when I was pregnant with my first child. I took a pregnancy test in August when myself and boyfriend broke up, but am increasingly worried that I may be pregnant and the test said negative because of the implant. What do you suggest? Do you think I could be pregnant?