There are a couple of reasons why a female dog in heat might become aggressive towards other female dogs:

Competition for breeding rights: A dog's heat cycle is essentially a signal to males that she's ready to mate. Females can pick up on these signals from other females, even if they're not in heat themselves. This can trigger a competitive instinct, leading the dog in heat to see other females as rivals for potential mates.

Hormonal changes: During their heat cycle, female dogs experience a surge in hormones like estrogen and progesterone. These hormonal fluctuations can influence mood and behavior, making them more irritable or even aggressive.

It's important to note that not all female dogs in heat will become aggressive. However, it's a common enough occurrence that it's best to be cautious when introducing a dog in heat to other dogs.

Here are some additional things to consider:

Prior relationships: If two female dogs normally get along well, a heat cycle might cause some tension, but it's less likely to result in a full-blown attack.

Presence of intact males: If there's an intact male dog around, the competition for breeding rights can be even more intense.

Spaying your dog can help reduce these issues, as it eliminates the hormonal fluctuations associated with heat cycles.

(http://www.autoadmit.com/thread.php?thread_id=5510465&forum_id=2#47534919)