Here are some additional possible reasons why a girl might ignore you after slow dancing with her:
*You made her feel uncomfortable. Whether intentionally or unintentionally, you may have done something that made her feel uncomfortable during the dance. This could include things like holding her too tightly, stepping on her feet, or talking too loudly.
*She was tired. It's possible that she was simply tired or feeling overwhelmed and just wanted to end the dance early.
*She didn't like the way you danced. While everyone has different dancing styles, if your dancing technique, in either leading or following the moves, falls out her comfort zone it can cause her not wanting to interact after.