Fear not, it's normal! They're so small, they don't hold body heat well and she's helping them out with some insulation.
I don't know how much you know, but it sounds like you're new to rat babies, so...
1. Check for milk bands. These are little white bands across the rittens' tummies that show they've eaten. Because their skin is so thin still, their stomach contents show right through! If you see them, good! Check later to make sure they're all still getting enough to eat, although it will get harder to spot them as they get older.
2. If you don't see good enough milk bands, or if for reason you feel like they might not get enough to eat, get some kitten milk and a dropper ready to step in and help Chloe out. In fact, this is a good idea to have on hand anyway, especially if there are more rittens than there are nipples. Because if she decides to stop nursing in the middle of the night, you're not going to be able to help her.
3. Feed momma lots of protein! Scrambled eggs is a good source, and rats really like eggs!
4. See if you can start handling the babies pretty much right away. Take momma out of the cage, though, for some free time, so she doesn't decide to get defensive. And only let the sessions last for a couple minutes at a time when they're this young, so that the babies can be attended to.
That's all I can remember for now, someone else will chime in, I'm sure. Good luck with them, and we want to see pictures!!!