What can I do about my weight?

I'm 5 feet tall, and I weigh 131lbs. i'm going on vacation with my 2 friends, and they're both TINY! One is 5'6 and weighs 115lbs, and the other is just about as skinny as her.... I do not wanna look fat on the beach next to them;I don't want to be that fat friend in all the pictures. At the beginning of this week, I weighed about 135.6lbs. I've barely been eating anything, limiting myself to about 600 calories a day. Yesterday I did really good, only eating a little over 400 calories. But I gained .8 of a pound!! And I was active the whole day, and I ate strawberries, broccoli with cheese, and a waffle. With 8 glasses of water like usual....How the hell did I gain .8 of a pound?! I'm leaving for vacation on in 3 days (Saturday) and idk what to do anymore.