I think I'm the opposite. I know when something is very wrong. Except it's not just due to the severity of pain.
When I had a blockage and then perforation, there was a lot of pain but not totally unbearable, yet I knew I needed to get to hospital right now .
When I had ileus, that was the most painful, and I knew from the level of pain something was wrong, even as some of the doctors were telling me there wasn't. (I was in hospital recovering from surgery, but the doctors did not suspect ileus at first because I wasn't vomiting.)
When I had an infection following another surgery, I'd been to see a GP to tell her something was wrong, that I was getting worse, not better. She said nothing was wrong, it was just that I'd had a major operation and pain was to be expected - a couple of days later, she was trying to admit me into hospital, because I had an internal infection.
The weirdest one was recently when I had an air embolism. I was calling out to the nurses on the ward that something was wrong, I was literally saying "something is wrong," but there was no pain and I couldn't think of any less vague way of getting across to them that I knew something terrible was happening.
Most of the time, I'm pretty much pain-free, or at least free of anything more than a lot of minor, nagging pains. Perhaps that's why it's easier for me to know when something is really wrong.