Constipation is marked by infrequent bowel movements, and passing hard stools. It also causes a great deal of strain during bowel movements. Nausea, however, is not a condition, but a symptom. This may be caused by several medical disorders, and constipation is one of them. The reason constipation and nausea are related is because in severe constipation, the food has nowhere to go but up thus, it triggers nausea and sometimes, vomiting.