Awww, if she suddenly became lethargic, not very active anymore, loss of weight, hanging her head off of things and sometimes breathing hard while being still, plus the cyanosis (blue colour from unoxygenated blood) of her extremeties, as well as a coolness to them, she might have CHF or congestive heart failure. The easiest way to find out is to give her a diagnostic dosing of something like lasix or enalapril or even both. IF she responds, her extremeties warm up, things just improve within 1-3 days, then she has a heart issue and will need to be on those meds for life.
IF there's no change at all, the meds will do nothing to harm her they just won't help and she doesn't have a heart condition.
Then you are looking at COPD (chronic obstructive pulmonary disease), pulmonary abscesses, scarred lungs etc.
Did she slowly regress, and act sick? Pneumonia is a true illness not just a condition. Tumours could be pushing on her lungs not allowing her the full capacity to breathe but the weight loss is not always indicative of this.
I would seriously try your rat on heart meds, since its the easiest way to "test" for it.