What Causes Pregnancy Symptoms While Being On 'implanon'?

Hi. I have a confirmed case of my Implanon being in my muscle...CT scan and ultrasound confirms. Scheduled for surgery next Friday for the removal. However, about three months ago I experianced first trimester pregnancy symptoms, which I attributed to the Implanon, considering I have experianced that before with my previous Implanon (which was inserted correctly and worked fine)...however currently I am experiancing symptoms similar to second trimester pregnancy (I have had two children before, now 7 & 8 years old.) With it being in my muscle I have been told that its effectiveness is decreased dramatically. They have given me a quantitative blood serum test and told me it was negative...however I still feel these symptoms (pinching and movement in my abdomen and also pressure on my lower rib cage. I am concerned that the blood test could have produced a false negative because of my situation, should I demand an ultrasound before the surgery, if nothing more than to settle my own mind? Extremely concerned because although I may not want to be pregnant at the moment, that does not mean I would want to harm a possible pregnancy.