Do my dogs know something I don't?

I have my two dogs with me, as well as my young, intact male’s sister. The sister (Lucy) was in heat just now. We kept them apart for four weeks, then went for a walk together last week. All was good.

Today, both my dogs are acting like Lucy is right in the middle of her heat. The young one keeps trying to jump her. I had to separate them to ask this question. What’s happening, any ideas? I’ve never observed anything similar. They played fine together last week, nothing unusual at all.