brown discharge

Here is my story: 2 weeks ago I wasn't expecting AF. The day before I was expecting it, I started with a dark brown discharge. Not enough for a tampon, but enough for a pad. This continued for 3 days, so my husband and I decided to have sex. And that caused my AF but only lasted a day or two with actual blood. Now the brown discharge is back and continuous. So I've had this discharge for almost 2 weeks. I went off birth control 4 months ago so I'm not sure if that's what's caused it. But I've also taken 2 BFN pregnancy tests, so it's not that either. I've also read that STIs and PID could cause this but I've been with my husband for 2.5 years. Please helppp!!