Most Helpful Girl
I think you have been so much together for a short amount of time, that it got a bit too intense. Now that you're aren't seeing each other more (as friends) your mind/body feels something is missing. I don't think it has something to do with love/feelings etc, you've just pulled yourself out of a situation, and you've sorta been codependent on this girl due to your intership, that you body is reacting to that. Give it some time and you'll be fine :-)
Most Helpful Guy
Maybe it's regret? You wished you had said something... anything and now it's too late? You know you can't do anything to change it now.
If you want my advice and I've been there man. Just learn from this... Learn that sometimes you need to say something. If you don't... you'll be left wondering all your life. And, maybe you even had a good chance... maybe not. But you wouldn't know unless you had taken some sort of action.