HELP PLEASE!! Late Period - Day 6

Courtney • Love the Lord & all four-legged things. On a mission w/ Mr. Benjamin Robertson since 11/03/12
My period is almost 6 days late now. I've been having cramps and every time I eat I literally feel so full that it's hard to breathe. I took a pregnancy test five days ago and it came back negative. 
I started having very, very light in flow spotting, but it's brown and isn't pink much at all. I am sexually active, but we're pretty safe. Before you ask- no, I am not of BC and we do NOT use condoms. Can someone tell me what this might be? I doubt I'm pregnant seeing the test came back negative and I'm spotting now. (Implantation bleeding would have been spotting closer to the start of my expected period, right?) 
I just need help. Feel free to ask questions. I'm not making this anonymous because I really need some insight. Thank you!