my yellow bellied slider turtle has been sick

my turtle is one and a half years old and has changed in behavior the last 24 hours.he yesterday left most his food in the morning and by 2pm throw up the food that he had eaten. we cleaned the tank out after this and he seemed to pick up, but after feeding him this morning he yet again left most of the small amount that i have put in.he seems very tired and doesn't seem to be swimming around like normal. please could you give me some advice of what to do?