How soon can I test after implantation bleeding

My lmp was on 11/21, I believe I ovulated near 12/05. I started having light brown spotting on the 15th. It was very light and almost flaky. It ended this morning. Could that have been implantation bleeding? I read implantation can take place 6 - 12 days after ovulation. If it was implantation bleeding ...how long do I have to wait to take a pregnancy test. I took one this afternoon and it was negative, but if I am just now spotting, is it possible that the hormones are still too low to register?

Also, I had a pelvic exam on Friday and the Doctor said my uterus was soft but she couldn't really tell anything else. What does a soft uterus indicate? The lab was closed so I was unable to have a blood test.

Usually preganncy causes a uterus to become firm.

You can take an 'early response' preganncy test about 4 days bofore a period is due.