My periods are always normal, never ever that light or short

So my boyfriend and I had sex on my birthday which was May 4. We did not use protection. We did it differently, he was on top this time. About 3 to 5 days later I started getting what I thought was "period cramps" and I went downstairs and went to the bathroom, and when I wiped (sorry if tmi) it was a little bit bloody, but it was brown blood. That lasted about 12 hours, but i didnt have anything on my pad, only when i wiped. I still have a little craming. I've been told that it called "implantation bleeding". My periods are always normal, never ever that light or short. Im not on birth control or anything. i've taken 5 pregnancy tests and they all came back negative. I just took the last one today, and it also came back negative. I don't know when my last period was though. My boobs are very tender. I have this "bubbly" feeling in my stomach, and ive been having a lot of gas. I have been getting these little headaches. I don't know what to do. So if somebody knows what this is, please let me know!! Thanks for caring!