Continuity mistake: During John and Riddick's final scuffle, once John's regains his gun and is aiming it at the camera, the gun has a visible orange plug in the barrel. When he then turns around and aims the gun in a different direction (though still facing the camera), the barrel is no longer plugged up.
Continuity mistake: After Johns dies and Riddick is talking to Fry you see in one shot that only the bottle that Fry is holding is lit up, yet in the very next shot you see that all three, Fry, Jack, and the Muslim kid's bottles are lit. (01:23:40)
Continuity mistake: When the guy with glasses (Paris) is chilling with his fan, there is some material stuck out between his right ear and his right shoulder. In the next shot when he retracts the fan, it's gone. (00:23:45)
Continuity mistake: When Fry is sitting by the giant bones, Johns approaches her drinking from a bottle and offers her a drink. When she takes the bottle, it has the cap on.
Continuity mistake: When Riddick powers up the ship the first time during the escape, he is seen with blood on his left ear. This blood was not there on any previous shots and promptly disappears in the next shot.
Answer: Yes, it is her period. Considering that he lived in darkness for years, it's possible that he smells it too. It's also equally likely he realized she was a girl and that the creatures were all riled up and put two and two together. But the first explanation fits Riddick's character better.
Grumpy Scot