Hi. Thanks for your query. The obvious cause for your stomach cramps, nausea, vomiting, can be due to PID- Pelvic Inflammatory Disease as you have history of green brown discharge. There may be vaginitis which has migrated to the pelvis cavity to cause PID. Since implanon is there, no chance of pregnancy, so not to worry about it. I would advise you the following: Get an ultrasonography of the abdomen and pelvis, tests of blood, urine and stool to rule any problem. Consult a Gynecologist, get an internal examination done and go for the tests and investigations and treatment as advised by her. You may need a treatment with an antibiotic, metronidazole, anti-fungal oral and vaginal tablets, good local hygiene. No sex till you are alright and ask your sexual partner to get examined, investigated and treated if you want the recurrence.