I think my young mouse is sick :[

Question:
 About one week ago, I adopted a new young mouse, about 5-6 weeks old. I kept her quarantined for about a day and then very successfully introduced her to my two older fancy mice. She has been doing extremely well getting along with the other two, and in the first few days, I didn't notice anything abnormal about her. However, I took her out of the cage today, in order to familiarize her with my hands more, and I started to notice a lot of things that are a little 'off' about her: -She has gotten extremely fat in the space of a week. Even my older ones have not gotten this fat, and the weight gain in a week period is alarming to me. You can even notice the pudge from above, rather than just from below/sides. -She is a lot more lethargic than the older two mice, and even when I put her in the mouse ball so that she has a lot of time to exercise, she doesn't like to move around a lot. -Her eyes always seem squinty. -Her droppings are green and runny, although the other two are digesting their food just fine. The other 2 don't seem affected at all, so I think it's ok for them to share a cage.. but if she needs to be quarantined, then I'm able to do that too. Please help :[
Answer:
 Here are a couple other thins, just for reference: 6/7 days a week, they're fed healthy 'lab blocks'. One day a week, they get a 'treat day' where I feed them a mixture of seed mix and Special K. They have had one extra treat of coconut cake crumbs and a piece of bagel. I put vitamin drops formulated for mice in their water, which is filtered, and replace it often. They have access to an exercise wheel I don't think the bedding is making her sick, b/c I only use a mixture of Carefresh and aspen chips. I clean the bedding whenever I notice smell and wash/deodorize the cage once a week. Thank you
Answer:
 Before you bought your female, did you notice if she was being held with male mice at the pet store? If so, the rapid weight gain could mean pregnancy. Usually the bulge appears quickly and will in fact make them very lethargic once that large. (Like any pregnant woman would be) I've had three mice so far that I've brought home and found to be pregnant. If she looks to be mostly chubbing out at the sides instead of a more all-over fat..Perhaps you should keep an eye on her, watch to see if she's nesting and if she is, be sure to supply her with lots of soft material and a small box. (If you don't already have one) Also, if you know for a fact that she hadn't been kept with any male mice at the pet store..try gently feeling her stomach for anything noticably firm. Mice are prone to tumors..but I'd assume that if she's rather large and only acting sleepy..that shouldn't be the problem. Although, my last female, who I also thought was just fat was recently put to sleep after her stomach was literally engorged with them. :( But she showed many other signs of illness other than groggyness. Finally, if you've labled out the first two..and still think she's just a chubby girl.. Try to keep her on a diet, like any animal..obesity is harmful. Buy a healthy seed mix without anything oily or fattening. (Peanuts, sunflower seeds- etc.) Also, throw in some healthy veggies/fruits that they'd all enjoy- but with moderation. An excersize wheel also helps a lot..Good luck!