weight loss/behavior change

Question
We rescued an approximately 2 year old Rex rat from a case of neglect about 5 months ago. Over the last couple of weeks she has changed, losing weight, not running around each day when we take her out, just laying on us and going to sleep.  She is definitely eating less but is eating and no longer runs on her wheel.  I tried to feel for any tumors but she really freaked out and I was not able to check well.  Is it possible she is depressed?  In reading about them it sounds like we should probably have another rat for a companion although she has never been with another one so I am a bit concerned about trying this.  Any thoughts would be appreciated!

Answer
Does she tire easily?  Do you notice her breathing heavier? She is older and she may very well just be happy to be in a happy home now, but a vet check up would be a good idea no matter what. If you need a vet please let me know and I will happily give you names.