AITAH For not going to a party?

In work I am usually not included with anyone. Because, most of my time I spend it on my desk and working. I cannot stand up and talk around because I am too busy. My coworker, she sits besides me. But she is always leaving her desk and go missing for minutes. Well, I was invited to her birthday party. I talk with this girl because she literally sits besides me. I wanted to consider my friend, but she rarely include me in anything. However, I am not convinced to go. I am not a party girl. I don't like drinking, or staying up late. The party starts at 9. Wow! So late! (Sarcastically) but for me it is late. I don't know. I even bought the dress and the shoes... because it's theme party. But, I am not convinced to go. AITA if I don't go? What you guys believe? Am I a bad person?