If your cycle is usually about 30 days (which it would be if you have your period on about the same date every month) then it is likely that you usually ovulate round about day 16 of your cycle (everyone is different but the average ovulation is 14 days before your period). Implantation would then occur about 7-10 days after ovulation. This means that implantation would have been prior to when your period was due. By my reckoning from what you have said, this would have been at the end of February/early March. To have it 6 days ago doesn't seem to fit with what you have said about your cycle. Some women do spot during pregnancy though so even though it doesn't sound like implantation it could be normal spotting.
As I said in my other post, I would have thought a test would detect a pregnancy by now however. They are usually pretty good from the day of your missed period. Yours was almost three weeks ago and so if you were pregnant, it is most likely that a test would detect it by now. Having said that - some women do not get bfps for some time.
Are you usually regular? Have you done anything different this cycle (come off bc, taken new vitamins, herbal remedies)? These can all interfere with our cycles. As can stress, worry, etc.?
If you continue to be concerned, it might be worth going to the drs for a check up.