I have joined this site in a desperate attempt to receive some help regarding a behavior change in our two cats after an incident the other night. They have been living together for almost 4 months now, they play and get along really well and we have never had to worry about leaving them alone together. The other night, I noticed there was a neighbor cat that had approached our sliding glass door (it was not open, so there was no physical exposure, only visual). Our older cat (12yrs) was two feet from the sliding glass door, ears back and growling at the neighbor cat. About two feet behind her is our younger cat (11months) also growling at the neighbor cat. All of a sudden, our older cat leaps at the sliding glass door in an attempt to attack the neighbor cat, and our younger cat immediately starts SCREAMING excessively while turning to run upstairs, away from the incident. It was a blood curdling scream that I've never heard a cat make and I've owned cats my whole life. Throughout the next half hour, our younger cat attempted to come back to the kitchen (where our older cat had been sitting since the incident) and when she saw our older cat, she growled, hissed, and then screamed and ran away. About an hour later, they ended up approaching each other in our basement, and they seemed to be ok. They even played with each other, and there were no signs of apprehension. Another half hour passes, and I hear my younger can growl and then scream at our older cat again. I separated them for the night, and the next morning, our younger cat growled when she saw our older cat so I decided to keep them separated for the day. Later that afternoon, I brought the little one downstairs and she began interacting with our older cat like normal again. We again kept them separated that night, and the next morning the first sighting of each other went as normal, and then all of a sudden the little one put her ears back and started growling at the older cat until my fiancé intercepted and started petting her. Then things went back to normal for the rest of the day. Then when we went to go to bed, my fiancé followed them downstairs for a minute, and once again the little one howled at the older one (keep in mind, the older cat has done nothing to provoke this behavior and has not growled or hissed, I actually think she's afraid of the little cats behavior towards her) and stopped once my fiancé intercepted again. We often go on weekend trips and I am extremely afraid to leave them alone together, especially at night when it seems like the trauma reappears in our younger cat. We bought Feliway plug ins, not sure if they are helping. I am terrified that she will be traumatized for a long time and we will have to separate them every night in fear that something bad might happen if we don't. Feeling so sad.