I did around the 6-week mark... a little late for implantation bleeding so I was beyond panicked! After a quick call to my OB, they went ahead and scheduled me that same day for my first OB visit with an u/s to confirm uterine pregnancy (I've had a history of ectopics).
U/s confirmed a beautiful 6w3d (I can't exactly remember) fetus with a heartbeat. Interestingly, my urinalysis came back flagged for a major UTI. I had no idea...I've never had one before.
My OB said the UTI could most definitely have been the cause for my spotting, but that there could have been other reasons too; she told that 1st trimester cramping and spotting is quite common, although I had never experienced it with my other two normal pregnancies. (I've had 8 in all and with my 5 miscarriages I definitely had abnormal bleeding.)
If you're really early...like around when you'd expect your period then it could be implantation spotting; it you're later than that...who knows the reason? Recent intercourse? If you are really concerned call your OB and they will get you in.
Best wishes, Valerie