Randomly throwing up food all of the sudden. How can I figure out why and stop her from getting sick?

My 2yr old flat coat ret. mix has been throwing up her food for the last couple of nights. She seems to only want to do it in the evening and around bed time. It has gotten to the point where she just does it without warning. Yes I am sick of cleaning up vomit but I am more concerned about why shes doing this and how I can get it to stop before she loses weight or becomes dehydrated. She hasnt eaten anything strange and she is not throwing up bile or anything odd, just dog food. Shes been on the same food since she was 1 and shes really slim so I know she doesnt just eat to be eating. Can you please help me figure out what is going on? She is on a rice and chicken diet for the day to help settle her stomach. Do you have any suggestions or ideas as to what is wrong with my princess??