it could be a healing crisis brought on by your body having so many nutrients to work with.
it could be an allergic reaction, because wheat grass is still wheat, which is an allergan, especially for people who have eaten bread all of their lives. If you have a compromised digestive system, wheat is very hard on you.
Also, wheatgrass and sprouts have protective agents that cause allergic reactions. I haven't been able to find a good reference on this yet, it was mentioned in another forum.
I myself have given up on wheatgrass juice and only take wheatgrass in my Perfect Food supplements. I seem to handle it okay once it's been partially processed. But others have had great success with it so...
Do you know how to do applied kinesiology, or know someone who does? That's a great way to find out if something is really and truly good for you, as the body does not lie and always knows. :) You could even just hold a glass of juice next to your abdomen and have a friend try to push down your arm. Then hold nothing and have them try to push down your arm again. Feel the difference - if it was weaker with the juice, then you know it's not right for you at this time.