If you used tampons during your last period, what you're describing sounds like a "lost" tampon! I've had that happen to me twice in my life and it's exactly as you describe it. The brownish, watery discharge and the smell............Oh good God, the smell was bad enough to gag a maggot!
When I went to my gyno's office and "assumed the position," before he even touched me, he said, "I can tell by the smell that we've found that tampax you forgot to remove 4 months ago!" And sure enough, after he went "spelunking" in there for awhile, he came out with this disgusting green, slimy, stinking............I better stop because the memory is making me gag.
I couldn't understand how it could actually get lost in there. I mean, the place isn't THAT big! But my doctor said it was very common. The string gets pushed up inside when we wipe and then somehow when we go to change tampons and don't feel a string, we assume we forgot to put one in, so we shove another one up there which pushes the first one as high as it can go and then the blood and secretions makes it all gooshy and eventually it sort of glues itself to the top of vaginal wall, especially during sex. The penis works kinda like a hammer and "pounds" it into place.
Anyway, from what you describe, my guess is a lost tampon. But you need to see your gyno no matter what as the discharge and the odor are not normal.
I hope you'll let us know what it was.
Peace
Greenlydia