Advice please. Weird period?

So I had unprotected sex on my most fertile days (About 8 days or so ago). I started having dark brown spotting on Sunday that was very light, a week before my period was due to start which threw me off because my periods are usually always on time and are super heavy. A couple days have passed and now it's more of a pink to very light red but light still. This doesn't feel like a period. Very little cramping and I've been feeling sick, with a headache. I just took a pregnancy test while I'm still bleeding/spotting and it was negative. I'm just concerned because like I said it's unusually light and a light color and looks somewhat like mucus. If anyone has had this happen or has some advice I'd appreciate it.