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It is possible that there was some physical discomfort that she was feeling (upset stomach, arthritis pain, etc) but it may not have been anything physical at all. Cats can display a behavior known as displaced aggression where they are upset or anxious about something and they displace it to someone (human or animal) else in the household. As an example, if a cat sees something like another cat outside and they get upset or anxious about it, they may act aggressive to the next innocent bystander they come in contact with.
If this keeps happening, you may want to have her examined by your vet to see if there are any physical signs of a problem and try to keep an eye on her habits like eating, drinking and elimination. Also try to keep an eye if she may be seeing other things outside that may be setting her off.
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.