The role of contractions
The uterus is muscular. During a period, prostaglandins help it contract as its lining is shed. ACOG explains that higher levels near the start of bleeding help explain why cramps are often strongest early on.
Pain may also spread to the lower back or thighs. Where it is felt does not, by itself, identify the cause.
Two different patterns
Primary menstrual pain occurs without an identified pelvic disorder. Secondary menstrual pain is linked to an underlying condition and can last longer or worsen over time. A clinician makes that distinction through history and assessment; an online symptom match cannot do so.
Use the pattern as useful information
Record when cramps begin and end, whether pain appears between periods and which activities it interrupts. Seek medical advice when the pattern changes or pain limits daily life. Warmth is a comfort option, not a test for whether the pain is harmless.