It depends on how intense it is and how much of it there is.
Gore: I'd say that as a guideline, if it graphically describes exposed internal organs, I'd call it R.
Language: If the characters are running around screaming anything that the automatic forum censoring would block out with asterisks at the top of their lungs in just about every chapter, it's R. Or well, actually, if they use the swear words in a more vulgar fashion than just as exclamations or emphasis, it's R, but otherwise not really unless it's got borderline gore and sex as well. Although if there is heavy language in the form of exclamations but no sexuality or gore, you might want to include a warning that despite the PG-13 label it has lots of swearing. I'd at least personally not consider censored exclamations anything worth R-ing the fic about, but the MPAA might disagree.
Sexuality: If you were a little kid and did not know about the existence of sex, would you just blink and not understand what you're reading, or would you go "Ewwww!"? If the latter, make it R. Basically, I'd say that if you can tell the characters are having sex/talking about sex/alluding to sex, but only because you're expecting it and can read between the lines, it can stay as PG-13. If you refer directly to the reproductive organs or what the act entails, I'd bump it up to R. If it contains the graphic details of who is sticking what where, it's NC-17 and you need to get it mod-approved.
That is assuming that what I mentioned is the only instance of questionable content in the story, so if you've got lots of it altogether in the story, you might want to bump the rating up anyway even though each instance of gore/sex/language is milder than described above. So if virtually every chapter of your story contains something being torn apart by something clawed, it might qualify as R anyway even though it doesn't have any intestines lying around.
But that's just my personal feelings on the subject. Other mods are free to disagree.