worried about my chin

I have 2 chins that are about 1year or so old. Both females and I was just wondering about one of them that gets tired reallyy fast. When I take them out to play the one popcorns and runs on the walls like normal but recently she has been doing it for Mabey 5-10 min then getting tired and sitting there and then when I put her in the cage she runs to her house and sits there and sometimes her ears get red like she's overheating but her fur is cold to the touch and the room is about 68 degrees if not colder . The other one never gets tired like this or ever gets hot but she never really jumps around like the other. There's been no change of diet or anything and the only difference is I changed from a wheel to a sauser and they don't seem to use it ever . That's also a secondary question ...how the he'll do I get them to use it hahah