Pet problem...could be serious i dno?

uhh, im not sure if this is cause for a big concern however today at like 4pm i noticed my dog chewing on something white (13 week old cairn terrier female). I tried to take it out of her mouth but she swallowed it before i could do anything...30 minutes later she started convulging like heaving, and she threw up a small amount of white stuff, no food pebbles just a white milky substance. She went and dranks ome water and seemed fine. However maybe 5 minutes ago when i sat on my bed to study (i have a final exam tomorrow) and began petting her i noticed she was shaking as was extremely scared as evident from her tail being in between her legs. Is she just shaken up or is this cause for concern.