What To Do About & Guest

I am invited with a guest to a wedding and am not currently in a relationship. I invited my best friend to go with me to the wedding since she is also friendly with the bride. Then a mutual friend who happens to be in the bridal party told me that I should not bring my best friend because the bride had the option of inviting her and did not. She told me that the bride only invited me with a guest as a formality and did not anticipate me bringing a guest. I am so frustrated because I don`t know if she is saying this because she has discussed this with the bride or if she is speaking from her own opinion. I am not sure what to do. Should I bring who I want or stick out the wedding by myself? Do I contact the bride and ask her if she would mind me bringing my best friend? I don`t want to offend anyone but I do want to go to the wedding.