Depends on location and history
Consider:
hCG
Consider ectopic pregnancy in any female of reproductive age
Urinalysis
CBC
Chemistry
Possible imaging:
Ultrasound
Appropriate for intussusception, ovarian/testicular torsion, gallbladder, pregnancy, appendicitis
CT
May be associated with 1/1,000 lifetime risk of malignancy
Abdominal radiography<unk> xray films are specific, but not sensitive. As such, they have very little utility in the workup of pediatric<unk>  unless concerned for a foreign body. Do NOT use films to "confirm" a diagnosis of "constipation," as this is not specific and may also be found during surgical emergencies (e.g. appendicitis).
hCG
Consider ectopic pregnancy in any female of reproductive age
Urinalysis
CBC
Chemistry
Consider ectopic pregnancy in any female of reproductive age
Ultrasound
Appropriate for intussusception, ovarian/testicular torsion, gallbladder, pregnancy, appendicitis
CT
May be associated with 1/1,000 lifetime risk of malignancy
Abdominal radiography<unk> xray films are specific, but not sensitive. As such, they have very little utility in the workup of pediatric<unk>  unless concerned for a foreign body. Do NOT use films to "confirm" a diagnosis of "constipation," as this is not specific and may also be found during surgical emergencies (e.g. appendicitis).
Appropriate for intussusception, ovarian/testicular torsion, gallbladder, pregnancy, appendicitis
May be associated with 1/1,000 lifetime risk of malignancy<unk> xray films are specific, but not sensitive. As such, they have very little utility in the workup of pediatric<unk>  unless concerned for a foreign body. Do NOT use films to "confirm" a diagnosis of "constipation," as this is not specific and may also be found during surgical emergencies (e.g. appendicitis).
Abdominal pain 
Emergent
Nausea and vomiting 
Necrotizing Enterocolitis
Nausea and vomiting
Volvulus
Diffuse abdominal pain
Testicular torsion