Americans don't really ask, "what are you doing?" It sounds like

a parent that caught their child doing something wrong.

Instead, Americans like to ask, "What are you up to?" which means "what are you doing right now?"

Or they might ask, "What have you been up too?" which is more like "what have you been doing recently?"

Even more casually, "what you up to?" or "wutcha up to?"

Sometimes people also say "what you doin'?" or very simply, "wyd?" (girls sometimes hate this one)