implantation bleeding?

Hi all, just wanted to ask a question. i have read about many people who have had implantation bleeding, does this normally occur before you af is due? probably sounds a silly question really but af not due for another 4 days, (if af comes on time, which it normally doesnt) had a small bleed, more watery than anything, earlier which has almost gone and was brownish (sorry tmi!). the thing is we have been told we have fertiity problems and i am worried i am making too much of this, when it could be just my af coming early. any ideas????