Thanks for the query.
As your pregnancy test gave negative result, there is less possibility for pregnancy.
Implanon can present with side effects like intermenstrual spotting, irregular periods
And some women may have amenorrhea
It can also cause side effects like pain abdomen, cramps etc.
So the symptoms you are having could be side effects of the implanon.
If you still have doubt regarding pregnancy, consult gynecologist once and go for blood test
If the test comes negative, the possibility of pregnancy is very less.
If the symptoms persist, go for ultrasound
Ultrasound helps in ruling out pelvic inflammatory disease
etc possible causes of pain abdomen and any abnormalities of the uterus can be ruled out.
After ruling out these things, you can continue with implanon with symptomatic treatment.