That seems like a lot of weight loss to me in just a couple months time. Unless the chin was fat before you got the wheel and you changed the diet. I would say a weight loss of more then 20 grams a month is too much (as in goes down and keeps going down). I would pull the wheel, and maybe make a vet appointment, something else could be going wrong with the chin. Weigh can fluctuate from day to day, but it should only go up and down about 5-10g. It is true some chins do just run nonstop on the wheel and can't have wheels for that reason, they run more then eat so they aren't replacing the calories they are burning. It's more commonly a problem in young chins, but it can happen to adults too.
Some other things to consider, how often do you weigh her? Was the weight loss a steady decline over the past weeks/months? or a sudden drop? or is the last time you weighed her in November and you just weighed her once now? Also time of day matters, did the chin just drink? pee? eat? poop? just wake up? just finished running around? those can all affect weight. The best way to notice weight loss is weighing the same time of day about once a week, if there is a noticeable loss weigh more frequently, like a few time a week to once a day to rule out a bad weigh reading do to having just emptied (peed/pooped) and/or activity level (just woke or just ran around).