Can't tell behavior function!

My chin has been laying down flat like flush against the ground all cute and potentially sad? the last few days, I think more than usual and I'm getting worried. Weather is not an issue it's back to cool even though I got an a/c for that one day we've had so far recently. He seems fine otherwise. He was jumping around when I let him out to clean his cage he was climbing all kinds of stuff trying to make a break for freedom and stuff, has been eating, "grooming" his "area" lol, nothing looks injured or anything, no unusual noises, no flinching or anything when I pet him, but he's been napping more, between all that good normal stuff and looks blah whether it's tired or sad or is that all sick? BUT, the weather has been gloomy like raining all day today especially and I've noticed that blahs him out (nerd behaviorist me took casual data to prove I wasn't being a crazy obsessed mommy lol). Has anyone had theirs have a few days like this and it just be a mood? He's never seen the vet (he's 4, ish) and I don't want to take him unless I really think I'm not being a hypochondriac, but I know they can get real sick real fast and now I'm like omg what if he lays like that for a nap and that's the end of it?! Advice, experiences, please anything welcomed! Thanks as always!