Hello, Thanks for writing to us. Brown discharge, weight gain, tender breast etc are quite common side effects of birth control Implant (Nexplanon) for initial user. As home pregnancy test is negative, then you have nothing to worry much. Implantation bleeding is ruled out. Other reasons are old blood, endometrial tissue, hormonal imbalance, stress/ depression, irregular diet habit, vaginal/ pelvic infection etc. Please consult with gynecologist for complete health check up with relevant investigation. Take proper medication if required. Maintain genital hygiene. Avoid mental stress. Practice safe sex in future. Good luck.