My only concern is that implantation happens between day 6 and 10 and you passed that time. Did you do a three day or a five day transfer?
WISH YOU LUCK!!!! I hope the BFP "sticks"
Some implantaion spotting is expected. You should however call to put your mind at ease and make sure the amount you are spotting is allright. Reduce your ativity if you can.
I think you should call and go to your doctor's office asap. Congrats on your BFP and hope you all the best.
xo,
marline