<human>: Started dating a girl in a close group of friends. When I did so, her best friend is acting all hurt and upset because of it.

Background: Best friend was interested in me a while back, (~5 months ago) and I said no. I was not interested in a relationship her. Fast forward to present day when I start dating her best friend. She acts extremely hurt and will not talk to either of us, even when before she told us that it would be completely fine with her if we did. 

Neither of us want to lose her as a friend and are very confused as to why she is feeling like this. Any ideas?

Provide a brief overview of the text.
<bot>: Starting dating a girl and her best friend is upset because of reasons unknown. No significant event has ever happened in between the best friend and I, and she has already said it was okay for us to be in a relationship.