Hi.
Thanks for your query.
Read and understood that you are getting pain/cramps in the lower abdomen.
Since you are on Implanon, 
pregnancy or ovulation pain is not possible. 
There are other reasons of pain in the lower abdomen like:
PID- pelvic inflammatory disease
Enteritis
Salpingitis, 
appendicitis
and so on.
I would advise you the following:
Get a clinical evaluation and physical examination by a 
General Surgeon, 
Ultrasonography of the whole abdomen including pelvis.
Tests of blood, urine and stool to see any evidence of infection.
Get a course of an antibiotic, 
metronidazole and anti-spasmodics so that you get a complete relief, rest of the treatment as per the reports of the tests. 
If there is a surgical cause, get operated.