She is 12 years old; that is the age of puberty when there is a sudden upsurge in female hormones. A risk factor is not only in female hormones, but it is greatest during the phases of hormonal changes. So the disease was just waiting to be triggered, and change of environment did its job, lots of sunshine, a complete change in diet are known to commonly trigger lupus(1). It is also quite possible that lupus could have been triggered by some vaccine she received before the visit, or due to some tropical infection (2).
Thus what initially looked a bit strange, completely fits into the well-known picture. Further, there are many studies comparing the onset of lupus in various age groups. All the studies seem to demonstrate that the onset of lupus in childhood or young age is much more severe as compared to adult onset. Cardiopulmonary disease, hematological manifestations, a disease of the kidney is more common in childhood onset of lupus (3).