I agree, it's probably old blood coming out with some discharge. Brown = old blood. It certainly could be part of your MP coming loose. If you lost the whole thing you would definitely know it. If you continue to lose it you will probably know it as well as it's a different consistency than then usual discharge, but it's possible to just lose a little bit of it at a time. Sounds totally normal though. It's nothing that I would be concerned about.
Hopefully things start for you soon! GL!