Help BC question

I got my nexplanon placed on the 29th of April during my fertile window only using Contraceptive Films(I'm married). My period was scheduled for the 12th of this month, just yesterday (the 19th) I got very very little dark brown discharge on my pad and nothing else the rest of the day today (the 20th) still very little blood with cramps in my stomach (nor like period cramps) I also took a test to day (with purple dye, I'm visiting England)and it came back negative. Any advice? Please and thank you.