I'm betting you'll feel stronger and much better in another few days or so. I recently had both an acute bronchitis (thank goodness not the flu) and a nasty little cold. I don't even have a brain injury (my son does) but it knocked me for a loop for a couple of weeks before I started feeling somewhat normal and could concentrate on studies or thinking about anything. December was pretty much a bust for me. Give yourself some more time and lots of rest, it's pretty early still.