What Causes Severe Constipation In An Infant?

Hi. My daughter is almost 8 weeks old and has not had a bowel movement in just over a week. The last one she had wasn t even substantial. I am now concerned because after I breastfeed her, she is excessively vomiting (flows like a river) and afraid she isn t retaining the milk. I am starting to wean her to Similac as well but the constipation started before this occurred. Her belly looks protruding on the sides; no fever and she is sleeping through the night, no issues. She is just fussy after she eats though. any ideas?