My kitten is having some trouble....?

She is about 10 weeks old, but she is very small we think she is the runt of the litter since all the other cats have gained and grown much faster then her. Lately she will eat and then throw up this white nasty spit with a small amount of food. Yet she seems to have no problems with her bowl movements and no diarrhea. She also will sit there and cry just before she throws up and after seems to be very congested, but that goes away after a few min.