Random Bleeding & Cramping

I’ve been on Nexplanon since March of 2021. I only bled the first month & had nothing after that. I’ve had sex ab once a week the past couple months. Every now & then i’ll start to spot about 2 days afterwards and it lasts anywhere from 2-5 days. I always take pregnancy tests even though i know it’s not extreme my possible, but i overthink quite a bit. Just recently, i started spotting yesterday. The last time I had sex was sunday. This time it was different because I am also experiencing cramping, and it is a darker red/black blood & it’s a little heavier. I haven’t taken a pregnancy test yet, and i’m tired of wasting my money on them.

Any thoughts as to what’s going on?