What is wrong with my Pug?

Hi, I have a 6 year old Pug, he's woken up a bit more snotty than usual, he seems tired. He hasn't been sick, he's had his breakfast. There's no coughing or runny eyes etc.
Could it be change in weather or?
any suggestions?

I can't get him to a vet until Monday...

Well he could be perfectly fine or he could be ill with any number of things...nobody can really diagnose him but a vet. If he's peeing and pooping ok, then I'd probably just wait it out and watch him. Look for discomfort getting up, going up/down stairs, jumping, can't get comfortable laying down, etc. Make sure his gums are pink and you haven't noticed any distressed breathing, right?

It could be a weather change, a bug bite, a treat he shouldn't have eaten, or just a bad day. If there's any issues with pee (make sure its a steady stream, make sure its yellow not pink) or poop (unusual straining, not being able to go) than I'd take him to an emergency vet. You don't want to let a UTI/crystals/blockage or anything else fester for another couple of days. If everything looks normal and he just seems in a funk then you're probably safe to wait 'til Monday.

Did he have a new treat or anything yesterday? Play someplace new where he might have picked something up?

Hope he's feeling like himself again soon.