Hi,
Welcome to ehealthforum.
False positive tests results are possible with contraceptive use, but more so with treatment with infertility medications. Women are known to get pregnant while being on Implanon or during switching over of contraceptive methods.
If you suspect to be pregnant, or if the repeat urine pregnancy test also comes out to be positive, then consider visiting your gynecologist for a thorough examination and blood HCG level test to confirm/rule out pregnancy.
Take care.