What could make scoliosis start progressing in one's early 20s?

[Breeann Wright]

I was diagnosed with minor scoliosis at 19 and told not to worry, that it wouldn't progress because I was done growing. It's three years later. My hips have noticeably slanted and I've lost a half-inch of height in the past 18 months. Anyone have any idea what could cause this?