Pink/Purle blob on screen

My LCD projector started showing a purplish-pink blob near the center of the screen recently. It takes up about 1/4 of the screen. It starts off working fine but after 5 minutes the blob starts appearing in the center, pretty small, and over the course of the next 5-10 minutes grows to the 1/4 of the screen size. Once the projector has been off again and cools off and then is turned back on, the process repeats... any ideas?