Missed period, weird symptoms

My boyfriend and I would make out and there would be a chance of ejaculate getting on or around my vagina. I used no forms of birth control. I missed my period last month, and just a week ago I took a pregnancy test which came up negative, but then I started what I *thought* was my period with severe cramps, and slight nausea, but very light bleeding that lasted for about 3-4 days. Is that implantation bleeding.? My abdomen feels odd and tight, like I feel the muscles protruding. My stomach doesn't feel normal and I'm worried something might be up. Any thoughts.??