My rat is very underweight...

Question
I know she's eating, so that's not the problem. She eats the rat food blocks. She also has a mix of dry pasta, tofu, Total cereal, Cherios, dried fruits (bananas and raspberries), dry oatmeal, sunflower seeds, and a low-protein dog food.

I give her treats a few times a week and she loves them. She also eats rather quickly. When I give both of my rats a treat, she finishes in half the time it takes for my other rat to finish. I don't understand how she's so underweight when she's eating as much as my other rat, who's kinda fat. She's also very small for a rat. She has TONS of energy. She absolutely loves to run and climb on things.

I'm just so worried about her weight. I figure if she was starving, she'd be rather lethargic. She's FAR from lethargic though. I can't figure out what's wrong. She seems completely healthy aside from her weight, so I've just got no idea what's causing this.

Here's some pictures of her and my other rat:
http://i275.photobucket.com/albums/jj293/irtreefrog/DSCN1576.jpg [i had to give her treats to let me take a picture]
http://i275.photobucket.com/albums/jj293/irtreefrog/DSCN1578.jpg
http://i275.photobucket.com/albums/jj293/irtreefrog/DSCN1580.jpg [looking for more treats]
http://i275.photobucket.com/albums/jj293/irtreefrog/DSCN1582.jpg

She doesn't look that bad in the pictures
But believe me, she's skinnier than she looks
Her stomach feels disgustingly small
She's the agouti, the albino is my fatty :P
(her fur looks a little messy btw because they both just had a bath)

She does currently have a litter of babies
They're a bit over 3 weeks old, though
They're starting to leave her and eat on their own
They love to eat the dry pasta ^_^
So I don't think they could be the problem...
She's always been smaller than other rats her age
And she's always been on the thin side
It just seems worse right now

Answer
How old is she?  Is this her first litter?

She really doesnt look that healthy as per her photo. She is pretty, but she looks frail.  

Do you allow them access to food all the time as self feeders? If not, do that for sure. Keep food down at all times. Also, I would like you to give her some soy milk or soy baby formula several times a day to lick from a saucer.  Also, Ensure, any flavor, even chocolate.  Offer her this several times a day as well.  Each day, twice a day, allow all of the rats to have some green peas. Canned are fine. Drain and rinse first and if you can find no salt, that will be great, too.
Allow her to eat as much as she wants until she walks away from the dish. Also offer some canned sliced carrots when you give them the peas. Adding high caloric drinks such as ensure should help with the weight but its also full of vitamins so this is good for her.   

I am thinking she has lost weight due to the babies taking from her but lets hope thats the problem.  

Do you have a good exotic vet for her? If not, just in case you need to see one, let me know your location and I will happily give you some names.

When the pups are around 4 weeks old its time to start to wean them. Be sure mom has a place to go to get away from them for a while. You will need to get the males out of there anyhow by the time they are 4.5 weeks of age so they should be weaned around that time.  If she doesnt start to gain weight after this, she should see a vet right away.