People really have different opinions on the age to give fresh greens to bunnies. In a way - I think it depends on what the breeder feeds some breeders introduce small amounts of green grass and let them eat along side their mothers as early as 4 weeks. Some breeders don't introduce greens until after 8 weeks. Some say not to feed greens until after 12 weeks.....
So all I can say is, whenever you give greens to a baby rabbit you do it really gradually, and only small amounts of anything new.
My advice for what to do now would be for the next 24 hours, only give a small amount of pelletslike 8 pellets(because many believe if there is an imbalance of bacteria in the gut the protein in the pellets feeds the bad bacteria so you cut back on them) and give lots of fresh hay and water for 1 day - if it doesn't seem better the next day it might be bacterial and you should go to the vets and have them do a stool sample.
Also, Bene-bac probiotics to build up the good bacteria, its very safe and you can buy it at the pet store. So Bene-bac would be really good.