To be honest, I'm not completely sure whether the Scripture passages on women's hair length are descriptive or prescriptive, but how long a woman's hair is isn't a core doctrinal issue.
Referencing Romans 14, if a woman feels it's sinful to cut her hair, she would indeed be sinning if she chose to do it. Conversely, a woman who sees no issue with having short hair isn't sinning by keeping it short (or even letting it grow long).