4 week old kitten! Help!!?

Ok so about 2 hours ago one of my cats 4 week old kittens started to vomit. Roo (the momma cat) diet has not changed, she has not received any medication or anything like that. All 3 kittens have been the picture of health until now, and its just the one. She vomits up curdled milk. About 5 minutes ago, I stimulated her (just to check, in case for some reason Roo hadn't) and after a minute or so of the kitten meowing like she was struggling to push something out, she pooped. It was solid at first, then it turned to a liquid. She is still nursing, walking and playing with her brother and sister. Any suggestions? I plan on getting her to the vet as soon as they open.