Dogs often eat grass and it can make them sick, so she may be trying to make herself feel better.
Depending on how far gone she is you should feed her a higher protein (maybe puppy) diet, but not until she is about six or seven weeks. Do not increase the amount but give her several smaller meals so she does not feel bloated. Once she has whelped you must give her some additional calcium to prevent eclampsia.
I must say I think you should have looked into this before mating your bitch. Breeding dogs is not as easy as some people seem to think and to have healthy puppies takes a lot of care on the part of the breeder. What really annoys me is that all pedigree breeders are being tarred with the same brush when most of us do all the right things, but some 'pet' breeders just go ahead and let their dogs get pregnant without planning and investigating all that is involved.
Thank you for that Ian, I didnt deliberatly mate her....I live in Spain and I have 4 dogs and 3 cats all of them have been done, apart for this one, the vet discovered she has a heart condition and he advised against the op. she has been having injections to stop her coming into season, but she suddenly became alergic, the vet stopped them, we have been careful not to let her out, but of course not careful enough...we think this is the goat herdsmans dog....