He keeps chasing her!!!

[buddyb]

I hope someone can help me! I have a pair of discus that been hanging out together for a long time. I am assuming it's a male and a female. They are adults. The one I call the bull, started chasing his mate around the tank. She looks so stressed b/c she's constantly looking out for him and when she starts to run, his right after her. Now, she hangs out on the other side of the tank and looks a bit darker in color. SHe eats when we feed them and then hides at the back of the tank, away from the bull. He chases some of the other discus but not as much as her. She doesn't look sick but always on the run from him. Could they be mating?
It's so odd b/c just a few days ago they were inseparable. Hopefully someone will know why...please help!! :confused: