Wy is my Red Eared Slider biting my Eastern Painted Turtle every time it goes near it?

I had my Red Eared Slider and my Eastern Painted Turtle in the same tank for about 7 years and they never bit each other! Now all of a sudden my Red Eared Slider keeps biting my Eastern Painted Turtle every time it swims near it. The 2 turtles were given to me All's i know about them is that the Painted Turtle was bought from a pet Store and The Red Eared Slider was found. I do not know the sex of the turtles nor there age. My Painted Turtle is 5 1/2 inches hieght and 4 inches width. The Red Eared Slider is 6 inches height and 4 1/2 wide. HELP ME PLEASE!!!!!