If the dogs are not spayed, then they will experience hormonal changes as they go into heat. We do normally see some aggression toward male dogs at the beginning of the heat cycle since they are not yet at the right time of heat. We also sometimes see aggressions toward other females to establish their leadership and "right" to mate. However, even when spayed, they will still try to be the dominant female in the home. The most dominant female gets special privileges such as eating first, getting treats first and affection first. If you try and treat them all equally, you may be contributing to the problem. The most dominant female should be the one you show attention to first, feed first and give treats to first. You will have to push the other one away or ignore her until the other dog has been shown the attention. If you do not do this, then the alpha female will attack the lower ranking female since she can't reprimand you.
I would discourage any aggression by your female at all times. There are other causes for sudden aggression in dogs such as hypothyroidism. You can read about this here. It is also worth mentioning to your vet to be sure this isn't an issue.
It may even be a case of the leadership among the females is changing. When one dog becomes ill or elderly a younger, stronger dog will often assume the top position. An older dog will usually not just submit without some sort of altercation and this can be an ongoing issue. It may also be that the younger dog senses some weakness (illness perhaps) in the older dog which is triggering the fights. Sometimes it will look like one dog started a fight, but with dogs a wrong look or movement by a submissive dog can trigger a fight.
Attacking can also be triggered by an illness in the older dog as well, so you may want to have an elderly check up done on her to be sure she does not have a medical issue going on. I would start making your dogs work via the Nothing in life is free program (NILF). It is outlined below.
I would also start obedience training so you can control them better. The following site is helpful in helping owners train their dog. Be sure and click on the link to the page on obedience at the bottom. and links on subsequent pages leading to detailed instructions.
Additional training can also help. Both dogs should be leashed and if one dog even looks at the other dog, a correction should be done. Any sign of aggression including a prolonged look, hair raised on the shoulders, a growl or even a stiff legged walk, should be corrected. A correction is a quick tug of the leash and a firm low toned "NO". Once you have done this couple of times, you should notice the dogs ignoring each other. When that happens, you will want to reward them for the desired behavior. Again, use tasty treats like the hot dog slices. This teaches the dogs that you WILL not tolerate fighting in YOUR pack.
You will also want to keep a leash on the females at all times initially to grab if they should disobey. Dogs like knowing what is expected of them and they love the little paper thin slices of hotdogs that I use for treats while training. Give this a try and see how it works for you.
In addition, if the situation is not improving using the techniques I've described, you may have to consult a professional behaviorist. You can usually find a behaviorist by asking your Vet for a recommendation or you may be able to find one using the following site.
Living in a household with multiple females can be an issue. You may eventually need to keep the two separate to avoid these problems. You can read about this issue here:
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.