
She may well be crying out if she isn't really ready. I do agree though, checking that she doesn't have a stricture would put your mind at rest.
When you say that the dog is on and off her, don't forget that it also depends on his experience. My boys usually just give a sniff, and won't waste their energy if she doesn't smell ready. Novice boys often get it wrong. So don't assume that he knows best! :-)