Hypothyroidism exacerbation: Given the patient's history of slightly underactive thyroid, the current symptoms of constipation, sleep disturbances, and decreased appetite could be indicative of a worsening of her hypothyroid condition. Hypothyroidism can cause gastrointestinal slowing, leading to constipation, and also affect sleep and appetite.

Gastroenteritis: The combination of stomach pain, constipation (which could be a manifestation of a gastrointestinal infection causing an initial diarrhea phase followed by constipation), sleep disturbances, and decreased appetite could suggest a gastrointestinal infection.
Irritable Bowel Syndrome (IBS): Although IBS typically presents with a longer history of symptoms, an acute exacerbation could mimic the patient's current presentation, especially with abdominal pain and altered bowel habits.
Anemia exacerbation: Given her history of anemia, if untreated or undertreated, it could contribute to her current state of decreased appetite, sleep disturbances, and possibly gastrointestinal symptoms due to reduced oxygen delivery to tissues.

Intestinal Obstruction: Although less likely, intestinal obstruction can present with constipation (or obstipation), abdominal pain, and vomiting. It's a surgical emergency and must be considered, especially if there's a history of abdominal surgery or other risk factors.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease or ulcerative colitis can present acutely and must be considered, especially with symptoms of abdominal pain, altered bowel habits, and systemic symptoms like sleep disturbances.
Colorectal Cancer: Though rare in this age group, it's a diagnosis that cannot be missed. Symptoms can include changes in bowel habits, abdominal pain, and unexplained weight loss or decreased appetite.

Celiac Disease: An autoimmune reaction to gluten, leading to small intestine damage, which could explain gastrointestinal symptoms and possibly contribute to anemia if there's malabsorption of iron or other nutrients.
Gastroparesis: A condition where the stomach takes too long to empty its contents, which could be related to her hypothyroidism or diabetes (if present), leading to nausea, vomiting, feeling full after eating only a small amount of food, and abdominal bloating.
Porphyrias: A group of disorders that can result in acute episodes of abdominal pain, neurological symptoms, and psychiatric disturbances. Though rare, they are important to consider in cases of unexplained abdominal pain and systemic symptoms.