I wouldn't worry about being pregnant necessarily as you are now bleeding quite heavily, if I read your post correctly? Yes implantation I believe does happen approximately 2 weeks after Mr Sperm succeeds and bullseyes Ms Egg and there can be brief/slight pain and a bit of blood at the time, not sure it would be brown though, brown blood is old blood not fresh I would expect it to be fresh blood and therefore red, but I'm being picky here. You may indeed have conceived and then spontaneously aborted which just results in a sometimes late and heavy bleed with a lot of clots, this has happened to me at least twice. I would wait another month and see what happens, you should get a period as normal a month or so from now. IF you don't, go get yourself tested without further delay to ensure yourself the most options going forward, good luck :-)