Do i have the right to be angry with my friend ? or am i being unreasonable ?

Basically, my *BEST* friend has just turned 16 and for her birthday (a month on) her other friend is throwing her a MASSIVE party on a beach in the countryside. of course my friend was told about this and has been part of some of the arrangements because she had to be available on the day- so its not a surprise party or anything. She told me it might be happening (as nothing was sure) two days ago and the date. The date is a random day in the summer holidays, so plenty of other available dates, and I told her i couldn't come on that day- so she said she'd look into changing it with the organiser. I know that sounds like im over-controlling, but she is my best friends and i would never have a party like that without her if there were other days around that time we could both do. Infact i was organising a party the other day and i had to keep rearranging the date because she was busy on SO many days. Im so annoyed with her now, because its become and official event on facebook and the whole year and others already invited and confiming/not confirming etc. I am assuming she has discussed it with the organiser since then as she wasnt sure of the date two days ago- yet no date has changed. she came to my house last night as her parents were out, and she mentioned it not once to me! she was even with me this morning and i actually feel slightly betrayed as if shes gone behind my back- which obviosuly i know is a slight overreaction. Well, I saw her begging someone who shes not brilliant friends with to come, and they said theyd maked changes to arrangements- but she didnt seem to care at all that her best friend wouldnt be there. So, i rang her now and she actually found the whole thing quite funny and was just laughing off sorry to me. It actually really matters to me that she made no effort to change it and doesnt even seem to care now. i know its so trivial- but do you think im over reacting ? (its a huge beach party and so many people are going.)