Dogs will often eat grass when they have an upset stomach, seemingly to cause vomiting. My Labrador, however, simply loves fresh young grass when coated with dew and will eat copious amounts with no problem other than when it reaches the other end of the gut which can necessitate removal with the aid of a hand in a poo bag (enough said!).
The risk is that a blade of grass may become lodged at the back of the throat, resulting in gulping and unproductive retching. I have seen cats in this situation, who can become quite distressed, especially after I have tried to look down the throat.
I think that a trip to your veterinary practice is needed. It is likely that your dog will need a general anaesthetic for your vet to be able to thoroughly look for a lodged blade of grass, which can flip up and over the soft palate.
Heart For Dogs 