Also thought this might be helpful for you:
This site had a wide range of different women sharing their true stories of implantation. Better to hear from actual mothers who went through it, than to only hear one type of answer:
http://wiki.answers.com/Q/Can_implantation_bleeding_be_heavy_like_your_period_or_is_it_usually_a_small_amount_of_blood
I had very dull cramping and nauseau 5 days before Christmas, breasts are larger and nipples are much darker. I even have a vein showing on nipple. My period was due 'today' and did not come. I am NEVER early and never late, always regular each month (28 day cycle). Yet this Christmas day, the cramps were still quite dull and I then had light spotting (the spotting was pink and only when i wiped, it then went to a bright (but light in consistency) red and then to a very faint brown then back to light pink and then gone. This all happened in 2 days, the 25th-26th. Today is the actual day I should have had AF and nothing. My husband is certain that I am. I had terrible nauseau today (this morning) and sat by the toilet for over an hour off and on, but nothing came...just the feeling of wanting to be very sick to my stomach. We are going to test tomorrow (first thing in the morning). Wish me luck! Good luck to all of you too!
Just wait to see if your period starts or if you have any more bleeding. If not take a test. Implantation bleeding normally happens a couple of weeks after fertilization. If you had any unprotected sex in the past couple weeks it is possible its implantation bleeding, but I would encourage you to just calm down and wait it out. You're probably fine!