I reread kitten-rescue, and it says there that a kitten might go once every other day. If she hadn't eaten in a while, that might be okay, since her body is going to use all the nutrients it gets to bring itself up to snuff.
If you go back there, www.kitten-rescue.com
, check what it says about how often to feed the kittens. Wake it if you have to, but keep the feedings on schedule. That's important. It also will tell you how much to feed. You don't want to overfeed either, which could cause lethargy.
However, I also recommend you take her to a vet for a few reasons. You want to make sure she is thriving, but you also want to have her tested for feline leukemia and other possible diseases, and checked for worms, parasites, etc., since you did find her outside. A vet could tell you closely about how old she is, which would also tell you when you can treat her for worms or other problems. Plus, if the vet feels she's closer to 4 weeks, it may be time to also introduce her to some solid food, but again, you can't tell until you have more informaton to go by.
Just make sure you do stimulate her enough, and long enough that she will poop if she has to. I think the site also tells you how to tell if she's "emptied."
Does her belly seem rounded and full? When she's drinking the formula, are her ears bobbing? Is she crying or sleeping peacefully?
If you enjoy carrying her around, and if you're good with your hands and making things, I know someone who was caring for an orphaned kitten who actually made a little "kitten papoose": A hammocky/slingy thing she tied around her neck and around her back that let the kitten sleep curled up in it next to her chest while she did some of the things she had to do, like be at the computer. Just make sure it's big enough so the kitten is completely enclosed in it so she can't roll out, and she would have to stretch to pull itself over the edge which would give you time to stop what you're doing and attend to her. Even some sort of cloth tote type thing. Then you could carry her around with you everywhere and be able to use your hands (which you'll probably still use to pet he most of the time anyway LOL)