Dysmenorrhea: This is the most likely diagnosis given Katia's severe menstrual cramps that are worse than usual, heavy flow on the first day of menses, and lack of relief with common over-the-counter pain medications like acetaminophen and ibuprofen. Dysmenorrhea is a common condition characterized by painful menstrual cramps, primarily caused by uterine contractions induced by prostaglandins.

Menstrual Migraine: Some women experience migraines related to their menstrual cycle, which could be exacerbated by or contribute to the perception of severe menstrual cramps. The presence of nausea supports this possibility.
Adenomyosis or Endometriosis: Although less common in adolescents, these conditions can cause severe menstrual cramps. Endometriosis, in particular, is known for causing dysmenorrhea that is not adequately relieved by NSAIDs.
Ovarian Cysts: Ovarian cysts can cause pelvic pain that may worsen during menstruation, although they are not typically associated with the menstrual cycle itself.

Ectopic Pregnancy: Although Katia's presentation does not strongly suggest pregnancy, any adolescent female with severe pelvic pain must be considered for ectopic pregnancy, especially if there's any chance of sexual activity. This condition is life-threatening and requires immediate medical attention.
Pelvic Inflammatory Disease (PID): PID can cause severe pelvic pain and is a significant concern in sexually active adolescents. The absence of fever and other symptoms does not rule out PID.
Appendicitis: While the symptoms do not classically suggest appendicitis (e.g., no mention of migratory right lower quadrant pain), any condition causing severe abdominal pain must be considered for this diagnosis due to its potential for serious complications if not promptly treated.

Mittelschmerz: This condition involves ovulatory pain, which is less likely given the timing of Katia's symptoms in relation to her menstrual cycle.
Uterine Anomalies: Certain uterine anomalies can cause obstructive symptoms leading to severe dysmenorrhea. These are rare and often diagnosed during evaluation for other symptoms or conditions.
Other Rare Gynecological Conditions: Conditions like uterine fibroids or rare tumors could potentially cause severe menstrual symptoms, although they are uncommon in adolescents.