Featured Image in Twenty Eleven Theme

I have been using the Twenty Eleven Theme for several years. I always set an image as “Featured Image” on my blog post and attach it to my front page as a “sticky.” When I published a new post tonight, instead of a thumbnail on my home page as “featured post” I got a banner size post (with no summary of the blog) and the image replaced my header image on the actual blog post page. Why this recent change? I have never experienced this before?