the_content_rss() is deprecated. You should be using the_content_feed(), but that is a secondary problem. Both functions echo content. What you want is the get_* variant– get_the_content_feed().
That is a general rule, by the way, but not a 100% consistent one. Many functions starting with the_ will output content immediately and there tends to be a get_ variant that will return a string (or other) for you to use.