Kereen,
First I would like to say I am very sorry about your daughter's recent health issues. I can relate to you as I have two children and both my dad and I had ulcerative colitis (he had an ostomy at 47, I had jpouch surgery at 49), so there can be a genetic link.
When I was about 22 and started having the exact same symptoms your daughter is experiencing (minus the vomiting), my dad immediately had me see his GI doctor. I had a sidmoidoscopy right in the office (no colonoscopy).
Very often at the early stages of UC, the disease begins in the rectum and works itself upward. She may not need a colonoscopy if her disease has just started, but since she has indicated this has been going on for awhile, I would not want to stall on getting her seen. Best of luck to both of you in getting to the bottom of her health issue.