We have had two cats for over 9 years now. One male and one female. The female had been and indoor cat until she was about 6 or 7, while the male cat was allowed outside. Slowly we began to let the female cat outside. Around the same time she became very aggressive around the male cat. We were forced to keep them separate from each other for a few months. Then it stopped and they were fine with each other. It has been at least a year since then and recently it has started again. Is there anything we can do to help her behavior? Is there anything seriously wrong with her?