Brown discharge??

I’ve had the nexplanon implant in my arm for almost a year (11 months), and my periods were somewhat normal until a few months ago. I’ve been having constant brown discharge whenever I’m off what i assume is my period, which lasted for about a week 2 weeks ago. But now it’s back to the constant brown discharge and I’m afraid to have sex with my boyfriend because I’m afraid it’ll gross him out. We tried it the other day, but i freaked out because there was the brown discharge on his dick when he pulled out. Here’s a picture of what it looks like (sorry if it’s nasty, I’m assuming this is the right group to post it in) my doctor says it’s just old blood, but how much old blood can my uterus have?? i just want a normal sex life with my boyfriend but this shit won’t go away, help?