This has happened to me several times and in my case it was always water retention associated with too much sodium (salt) intake.
Stick with your plan and it will come back off. It is not fat.
You have to consume 3500 calories above what you need to gain a pound.
5 times 3500 = 17,500 so if you did not eat 17,500 calories above your needs in one day, it is only water retention.
Easy to say but I get bummed out also when it happens.
Remember this too shall pass.