Irritable Bowel Syndrome (IBS): Given the patient's complaints of bowel changes, nausea, and vomiting, along with a history of pelvic pain and psychological history, IBS is a plausible diagnosis. The absence of findings on the pelvic exam and the planned transvaginal ultrasound further support this, as IBS is a diagnosis of exclusion.

Gastroesophageal Reflux Disease (GERD): The symptoms of nausea and vomiting could be indicative of GERD, especially if the patient experiences these symptoms postprandially or at night.
Inflammatory Bowel Disease (IBD): Although less common, IBD (including Crohn's disease and ulcerative colitis) could explain the bowel changes and should be considered, especially if there's a family history or if the patient has other systemic symptoms like weight loss or fever.
Functional Dyspepsia: This condition, characterized by recurrent upper abdominal symptoms, could contribute to the patient's nausea and vomiting, particularly if related to eating.

Ovarian Torsion: Although the pelvic exam was normal, ovarian torsion is an emergency that can present with nonspecific symptoms, including nausea, vomiting, and pelvic pain. It's crucial to consider this diagnosis, especially if the transvaginal ultrasound has not been completed.
Ectopic Pregnancy: Despite the lack of specific findings on the pelvic exam, ectopic pregnancy must be ruled out in any female of childbearing age presenting with pelvic pain and nausea, especially if there's any chance of pregnancy.
Appendicitis: While the symptoms might not be classic, appendicitis can sometimes present with nausea, vomiting, and bowel changes, particularly if the appendix is in an atypical location.

Gastroparesis: A condition characterized by delayed stomach emptying, which could explain the nausea and vomiting. It's less common but should be considered, especially if the patient has a history of diabetes or has been using medications that can slow gastric motility.
Celiac Disease: An autoimmune disorder that can cause gastrointestinal symptoms similar to those described, including bowel changes and nausea. It's less likely but should be considered if other diagnoses are ruled out, especially if there's a family history.
Endometriosis: Although the pelvic exam was normal, endometriosis could explain the pelvic pain and bowel changes, especially if the patient's symptoms are cyclical. The diagnosis often requires laparoscopy for confirmation.