I Have The Implanon As Contraception. I Had An Operation

I have the implanon as contraception. I had an operation 6 weeks ago in which I had 2 doses of antibiotics. I haven't had a period in near 3 months and have been feeling a bit nauseous. Missed periods didn't bother me as they are unpredictable with the implanting. I have done 5 pregnancy tests in which 2 have come back positive and 3 negative. Is this possible that they could be false positives? I am also taking 75mg amitriptyline a night so if I am I presume I must come off these asap?