Difficult to say, not much information here. Has your dog eaten anything different. If mine eat chicken or anything with chicken in it, they are sick. How many times has the dog been sick? Is he/she drinking water? Perhaps if you can give more information, someone here can help you.
It could be something simple or maybe more serious. If its persistant, the only answer is a trip to the vet ! You dont want to leave it too late, if it is more serious.
Please take your dog to the vet. As most know on AB,we lost our beloved old labrador last week.She started off by being sick and only drinking small amounts of water. Turned out all of her major organs had given up due to her age. I dont know the age of your dog and I hope its nothings serious but please see the vet.