I Am A Little Scared

I am worried that my cat is sick. How do you know? You can ask him questions? He is eating and drinking normally which I assumed would be the first clue to something wrong. But he is acting out of the norm. I have looked for puke and haven't found anything. Generally he spends most of the day downstairs with me. When I go upstairs to get something or to go to the bathroom he waits at the bottom of the stairs just to make sure I am coming back down. if I spend too much time upstairs or downstairs, he follows me. But for the last two days he has been upstairs by himself in the dark on the bed. I can't really afford to take him to the vet and I go back to work tomorrow so I won't be home to notice him being out of sorts. If after tomorrow he is still not himself, I will call to make an appointment. Hopefully I can get him in by the end of the week.