Depending on where he is it could be his symptoms never stopped, they might be coming back with some light biking, or maybe he's running and waking up with headaches. Without specifics all we can tell is he isn't able to physically exert himself without symptoms returning.
Luke Kuechly missed 3 1/2 games
This obviously isn't good news for Shields now or his future prospects since he'll likely be more susceptible to being concussed again. I would guess the more severe the concussion the greater the susceptibility but as PackerDNA posted, we're still learning about concussions. From the less important football point of view, Thompson's use of the first two picks in the last draft looks prescient.
I was talking about it because I find it concerning for his health, just seems like a really long time to be dealing with the concussion symptoms. I more concerned for his actual health than whether he is able to play or not.
The other things it makes me think of is wondering how many times I had a concussion when playing football in high school. The amount of time my bell was rung on a collision and continued playing is at the rate of at least 2-5 a game.