18 month old not eating

Hey guys so my son has his days where he eats everything in sight and some days he barely eats & these past few days he has not been eating good. He will eat breakfast some what & then he will have a few bites of banana or some grapes or blueberries & then for lunch or after lunch he doesn’t want anything same thing w dinner or snacks he doesn’t want anything he will take one bite and then he will spit it out. He usually loves water too but he also has been iffy about water so i am concerned something is bothering him, he has no fever nothing like that. He does have an appointment next week with his doctor but I wanted to know if this has happened to anyone w their LO.