I've had 2 early m/c's.
My first one was at 6w2d. It started with bleeding at 5w5d that wouldn't stop. It was bright red, and wasn't terribly heavy.....but it just wouldn't let up. Finally, at 6w2d, it all ended. I'd been continuing to temp (out of curiosity) and on this morning, my temp went below my coverline and I knew it was all over. I called in sick and lay on the couch all day with LOTS of bleeding and the worst cramps I've ever had (besides labor).
My other one was at 5w5d. I'd been having a some brownish spotting for a couple of days before, but didn't think much of it because there weren't any cramps. On the day it ended, I got some really horrible AF cramps (down low, central, and twisting) that wouldn't stop.
Then, later that afternoon, I got a big gush of bleeding and that was it. I had a long AF-type bleed (for about a week and a half) after that.
When I get cramps now, I pay close attention to them. I do get some that are down low and central (cervix focused), but they're always fleeting - they only last a few minutes, and then either move somewhere else or stop altogether. Plus, they're never debilitating. The cramps I had during a m/c were horrible - doubled over, need a heating pad/ice pack sort of thing.
VERY different to the normal, pg cramps.
Just for the record, the other m/c I've had was at 11w5d. The baby had died around 8-9 weeks and I didn't know it.
We couldn't find the h/b, and I went in for an u/s. I had NO signs that there was something wrong - no cramps or bleeding. Even for a few days after we found out, there was nothing. In fact, I even continued to have mild pg signs - smell sensitivity, etc. My ms was never bad with that pg anyway, so the mildness of my post-m/c signs wasn't an alarm. I elected to have a d&c because we were going on vacation, and I didn't want to wake up one morning while we were away to a nasty surprise.