I put on at least 2 pounds a few days before the TOTM, and also around the time of ovulation. I was on course for my 10% this weigh day, but I am due on Sunday and I am starting to see it going in the wrong direction already! And in the last 2 weigh-ins I stayed the same then lost 5 pounds, all due to water retention.
If I eat something salty (eg chinese) at the same time, I can put on even more!
To gain 5 pounds of genuine weight in a week, you would have to be eating about 2,500 calories A DAY over your needs, and if you are on plan, even with a few drinkies thrown in, you would find that tough to do!
My bet is, for some reason you have masked your weight loss and next week you will drop it again (have you been going to the loo enough? I don't do red days, but eating more meat and less carbs could have slowed everything down?)