Nausea is a common symptom of many different health problems -- it is a signal from the brain that you may have just eaten something unhealthy.
Possible conditions to watch for in nauseous dogs include:
- Dental infection
- Mouth Ulcers
- Overeating or eating too fast
- Chronic renal failure
- Gastrointestinal Disorders including Inflammatory Bowel Disorder 
- Ingestion of noxious or toxic substances 
See Renal failure for information on treating nausea that can be part of CRF.
- Effect of Renal Failure on Gastrointestinal Physiology in Dogs Lefebvre, H. P., et. al., 2001, WSAVA
- Poisonous plants for cats and dogs About.com