I just got two girl sister rats a week ago. They are 8 weeks old. One of them, Lisa, is doing really well, she's been getting bigger, likes to explore the cage, and comes over to investigate when I put my hand in the cage. The other one, Louise, is about 3/4 the size, is more timid, and also super low energy and sluggish. She isn't as curious as Lisa, and prefers to just curl up in a ball in a cozy corner of their cage all the time. She only perks up when I offer treats. What's going on? Do I have to worry about her? I already am. She is so sweet, I already love her so much. What is going on???