Is my Female Rat Sick? Not moving properly?

I bought a female black and white hooded rat about a month ago. When I got her she was perfectly healthy, no runny nose or eyes, and no sign of RI or anything of the kind. She looked and smelt clean. She lives with another hooded rat, tan and white, and today when I was cleaning their cage I noticed that she wasn't moving normally. It seems like she is having a hard time balancing her back end. She will he walking then her back legs will stumble and sometimes she will even fall onto her side briefly, then correct herself. The other rat she is living with is perfectly fine. I bought both of the rats at different times from different places. She isn't losing weight, and is acting like her normal self, except that she is moving a little slower than she use to. It's kind of like she is starting to lose control of her back end or something? What's going on? The pet store said she was only 6-7 months old.