How can I ditch this family reunion...?

I HATE HATE HATE visiting my mom's side of the family, and she knows this. She always waits to tell me at the last minute and tries to act like she told me when she found out. I HATE HATE HATE how loud they are and how my mom always acts super fake around them. The whole time I'm there all I do is stress out. I always seem to come second. A few weeks ago I had a massive 3 day headache from my concussion and she still dragged me to a stupid family function instead of taking me to the doctor like I was telling her I needed to. I'm afraid the stress of having to go to this reunion will bring on another headache and all I wanna do is rest and get better. How can I get out of this? Please help me!