Spotting, implantation, early miscarriage??

So my last period was the 29th to the 5th of March. And I had sex with my boyfriend on the 6th and 10th, and about three days ago (today being the 17th) I noticed some thicker brown discharge accompanied with abdominal cramps and some lower back pain. I'm 19 and my periods are usually pretty regular and I've never had spotting before. I took a pregnancy test yesterday and came back negative. Curious to see if anyone else has had this happen and what it ended up being???