Rat is weak and has no energy

Question
My rattie is about 1 year old. 3 days ago i noticed she doesnt come to greet me when i open the cage (as she usually did). she's lying down all the time, and when she walks around she looks so weak that she's sort of crawling almost. she eats small amounts very slowly when i feed her. i feed them things like healthy cereals, frozen vegetables, healthy bread, dog food pellets, and sunflower seeds.
the other rat in the cage is still 100% healthy.
I took her to the vet - and they didnt do a lot of tests on her..except for weighing her and took her temp. all was fine.
sometimes it seems she stuggles to get into a comfy position..and will turn around a lot - like she's uncomfortable. She's also
dirty from the red rat mucus - from stress. she's not aggressive..just seems very weak! she's not tilting her head, and her ears are clean. no bumps or tumors that i can see.
what else could it be? her breathing seems normal to me..but im not sure. she was exposed to a draft for a while when the cage was standing door for a while.. pls help!

Answer
Is she able to use her front paws when she grooms or grasps food? Try feeding her baby foods, see if she licks it. You said you feed her, so does that mean she doesnt feed herself?  Stop with the sun flower seeds, bread, and please check the protein levels on the dog food.
Is she drinking normal and urinating normal too?

Check her extremeties. Are her feet and paws cool to the touch? Do they appear pale, blue or purplish tinged?  Is he tail limp or does she carry it high?

Did the Vet offer any antibiotics at all? Taking a rats temp is pretty worthless. They dont usually have fevers like other animals because they lose body heat through their tail. A vet that knows about rats would know this.  Vets that dont know enough about rat care cause more harm than good.  What kind of vet saw your girl?  I am thinking you need another vet fast.