My left side of my head has been hurting since I started taking metronidazole and since I got my implanon, what is this?

Unilateral headache. Both Metronidazole and hormonal contraceptives including Implanon can cause headaches. But if you are having one-sided headache, which you have never had prior to starting these two, i would definitely go see a doctor. Among serious side effects of Implanon and other hormonal contraceptives include venous thrombosis, thromboembolism and stroke!