It's stress because you haven't adjusted to the situation. You have very quickly expanded your role but you are only part way to becoming manager at your current level. This is ok, the important thing is to recognize the problem and start working on what you still have to do to finish the transition.
Ask yourself, what it is that made your superiors put more responsibility in your hands. Find the capability that made you diffirent from your coworkers. Hold on to that, this is your differentiator. It's too easy to forget what is your differentiator when you advance quickly.
Recognize that you are now a manager and you are responsible for deciding what you are doing yourself and what you are delegating. If you are below VP level you will have to delegate almost everything even if you think you have to do it yourself. Try doing only one important thing each day. This important thing may be preparing speach, presentation, focusing on helping a project behind schedule, etc. Delegate everything else recognizing your limited time.
Recognize that, being a manager, your biggest responsibility is as enabler for your people. Empower your people, make them feel important, make them feel you trust them, and delegate things that are important and you may have hesitated delegating up to now. Hiring plans, work estimates, gap analyses, etc, do you think you have to do it all by yourself?
Find some management advice. As a fresh manager myself in very similar situation to yours (large part of organization and very important projects bestowed in very short time from my coming into organization) I like to listen to management audiobooks like Harvard Business Review and the likes. There is a lot of very good advice but the most important part is that you are actually aware what can go wrong in your current role so that you can recognize the problem and act on it before it becomes full blown disaster.
Most important, if you think something stresses you, stop and think. Don't let the stress overcome you, but spend on trying to understand what part of the task is stressing, why is it stressing and what could be done to remove stress. Don't let the downward spiral take you.