irratic swimming behavior

Question
I have a 75 gallon cichlid tank. My fish are acting really strange. They are a little more aggressive than usual and are yawning, flashing and shimmying. I have tested the water for amonia, nitrate and nitrite and everything appears to be perfect. I did however just get over a high nitrate/nitrite problem. Any ideas?

Answer
Hi Wendy,
 Before I can have any ideas I need to what exactly you mean by acting strange. My only guess from what you have told is maybe it is the time the year where there fish most mate. Otherwise they could just be males being males.
           Sincerely,
               Samantha