I think that Stephanie explained a little bit about it on her website. I think she said something along the lines of that when Bella has her cycle it isn't the same to Edward because it's "dead blood". It's more like waste material than fresh blood. Probably doesn't have the same pull for them.
But I could imagine that they would still know who is cycling because I'm sure it has a faint smell none the less. Just like they can smell the difference between clothes fabrics, I'm sure they can smell hormonal changes. Ever notice that animals can smell when other animals are in heat or cycling? It would be the same thing. But probably not a draw to them to attack.