Why is my cat all of a sudden very lethargic and lost his appetite?

my cat bucko is a year old, hes always been very healthy and he LOVES food, everyday when i feed him he goes crazy while im fixing his dry/ wet food mix, as he has gotten older he is not as playful as he used to be but he still plays with my 2yr old chihuahua all the time, but over the last 3 days he has drastically changed and im not sure whats wrong with him. he is very very lethergic, shows no interest in doing anything at all and wont eat more than a couple bites of food a day. im super worried, i love my kitty a lot but im a new mother of a 3 month old baby on a very tight budget so i really cant afford to take him to the vet. any suggestions on what might be going on and what i could do to help bucko