It's of course possible but very, very, very unlikely. Implanon is very safe, only fails if you happen to be very overweight and the hormone isn't strong enough, or if there was a problem with the insertion.
Considering your periods stopped when it was inserted, I would say that your implanon is working correctly.
Spotting is normal - some women spot constantly on implanon. All the symptoms you've described can be related to the implant.
So, I can reassure you, but if you're still worried, only thing that will reassure you for certain is taking a pregnancy test, which I'm sure will be negative for you.