I don't think you did anything wrong. Our bodies react sometimes in ways we don't expect. She sounds a little immature/inexperienced not to know that guys don't always know when they are going to come, and if she likes giving BJs, what happened was a possibility.

One of the keys to good sex is communication. It's awkward until you get used to it but you need to learn how to power through the awkwardness. Talking about it at a time when you aren't doing anything sexual is the way to go. I would try to see her and talk it over. Tell her you didn't intend for things to go the way they did, that you like her a lot and hope she is willing to continue seeing you. Don't drag your friends into it, it's private.