Cat suddenly lethargic and hiding in closet

Hi all, my boy Milo was himself last night and suddenly all day he went into the closet and has stayed there all day. He didn’t get up for his breakfast or dinner which is very unusual for him. We tried bringing him up food and water but he’s not interested, and won’t even react with treats. He doesn’t have vomiting or diarrhea either, he’s just acting very tired. Only other symptom is that he had a very stinky poop in the morning. Is it possible he caught a stomach bug? Is this something more serious I should call the ER about asap or should I wait to see how he is doing tomorrow? Thanks in advance for any advice!