Sudden behavior change

I have 2 male rats, about 9 months old. We went away for the weekend and when we got home, one of them (Mack, rex fur, healthy weight) is acting different. He is normally very active, won't stay still to cuddle, wants to explore everything. Now he is sitting in one place and cuddling, not very active, whether in the cage, free roaming, or being held. He mostly just sits in place.

I wouldn't call him lethargic, as his movements are normal when he does move and he is staying awake. No wheezing, no signs of injury, eyes are bright, eating and drinking normally, no unusual poo or blood in the cage, no noticeable lumps on his body. When we get him out, he moves a bit more after 10-15 mins. He has never fell asleep on me, even as a baby, but last night he did.

Is this just because its winter? (The room got close to 60°F overnight) did he just get depressed from missing us? I wont have the ability to take him to the vet for another week, but if its an emergency I can apply for care credit.