My cat's acting strange today, and he seems to be in some pain but I'm not sure where it's coming from. He's extremely lethargic and has been lying around all day. It's like he feels heavy. It seemed like his back paw was hurt, but I've been watching him and now it looks like it might be his front paws. I felt them each and he made no response and didn't cringe or anything so maybe I'm imagining that. He's also not eating.. I got him to drink a good amount of water though.

There's clearly *something* wrong with him, either he's sick or hurt.. but I need to know if this is an emergency situation or a tough-few-days-at-home situation. I can't at all afford to go to an after-hours clinic unless it's the very last resort and if that's the case I'm going to have to not pay some of my bills this month... which I'll gladly do if it's really truly necessary.
So I know the usual answer is 'take him in just to be safe' but do you think I can let this wait until tomorrow?

It sounds like it can wait. If he hurt himself jumping or playing or something, you'd expect him to be lethargic and eat very little. If there's still something wrong tomorrow, then I'd definitely get him in for a check up. But it doesn't sound urgent from what you've described.