Spotting or something else going on.?

Liz

Ok, lady's

Well I just got my period last month on Feb. 20th for almost a week, & after my period finished; I had sex with my bf, & since then I've been having like a light period I'm not pregnant I already took a pregnancy test my birth control is the nexplanon but I'm wondering if anyone else has had this happened & what's causing this.?