Spaying after the first heat MAY help with her growing body (mostly in larger dogs) but cancers are greatly greatly reduced.

Signs of going into heat may include one or more of the following and possibly other behavioral signs: swelling, bleeding, the nipples may enlarge, she may be more clingy or more distant, not wanting to listen, wanting to roam, wanting her butt area touched more, putting her butt in other dogs faces.

Even though your male is neutered he still can easily notice the signs & smells she produces, he may start to mount her & be close to her, follow her.