Usually them making a nest that early signifies they are not getting sufficient heat. Reading that you've only had them 2 1/2 weeks, you would already not miss that they are pregnant. A rat's gestation period is 21-23 days. If they looked like they swallowed a tennis ball, and are making a nest, that pretty much tells you that you're about to get some pinks. Since you are uncertain, continue to feed them as normal, when they give birth, you'll need to feed a little extra because they have to produce milk, or they will eat their young due to lack of milk production. Rats are very interesting, if the mother doesnt feel that she can raise her young, they wont survive, or threatened, she will usually eat them. When or If she does give birth, don't touch them or stress her for the first 3-4 days unless your feeding. I don't know how you feed your rats, or what you house them in, but you want to make it as stress free as possible for her. If you see her swelling as said before, move her to her own tub/enclosure, or move the male. If I have more than 1 prego, I will put them all in a tub together. If I have 1 out of my 5 females pregnant, she gets her own tub. Ive been breeding rats for about 1 1/2 years, so I have a pretty good idea of the behavior a prego rat shows. Im not by all means an expert, just figured I'd give you what Ive learned over the time Ive been doing it. Anymore questions, let me know.

Good Luck!

Trey