As stated above, yes. Cut grass is not healthy for your horse and is dangerous for them to consume.
Also as far as your horse's changed behavior, I have gone through this as well with my mare. That was because before I was severely injured, all I did was work with her every time I went out there. Of course I groomed her and all of that, but I also worked her. And hard. Mostly just lunging, but I made her sweat to keep her toned. After about a month of just doing this, she became a pain in the butt to catch out in the pasture, she'd play the "catch me if you can." game.
Once I got injured, I went out there as much as I was able to, and just groomed her. That was all I did. I just spent time with her. I just sat outside and watched them graze. Since I've gotten somewhat better, I've kept up with just grooming her, and then I work in a little bit of lunging every day. I up it 5 more minutes every work out. You just have to work on your bond, you might even have to re-heal a bond that may have been broken.