I would probably feel hurt at first but then I'd try to look at it from her perspective. She was probably hurt that you didn't join them. I'm guessing that she felt that you should have joined them because of your friendship and since you didn't she's hurt.
I suggest that you talk with her, using I statements, telling her how you felt. Include that you want to be friends and want to resolve this issue.
In response to another post's comment about it being inappropriate for parents to take their child to celebrate their 21st birthday, I strongly disagree. It's very appropriate especially if the child and parent's are close. My daughter's Dad and I took her out for a drink to celebrate her 21st birthday. It felt like we were acknowledging that she is now of legal age to drink. Taking her for a drink was a rite of passage.